When choosing the transmission for your swap, carefully consider its location and how it will mount to the chassis. In the stock position, older GM transmissions such as Muncie 4-speeds, TH350/400s, and 2004Rs mount to their original location in the vehicle. For 1968–1972 cars, the stock crossmember fits and bolts into the correct location along the original nine-bolt pattern.
However, 1967-and-earlier cars have a four-bolt transmission crossmember pattern on the frame, limiting the options for the stock crossmember. Your engine position and transmission choice may require a modification to the factory crossmember or replacement with an aftermarket unit.
Another solution is the sliding transmission mount from G Force Performance. This special transmount bolts to the transmission and allows for up to 2 inches of travel fore and aft to reach the crossmember. These are particularly handy for swaps using sliding motor mounts; fewer mounting bolts means fewer options for mounting positions.
The late-model transmissions, including the manual Tremec T56 and GM 4L- and 6L-series automatics, are different. The T56 is a very long transmission, so fitting this into any A-Body with the stock setback requires fabricating a new transmission tunnel. The stock crossmember can be used in 1968–1972 A-Body cars with the stock setback adapters. Many owners, however, are not comfortable with or do not possess the skill set to fabricate a custom transmission tunnel. Cutting and welding in new sheet metal and chopping up the floorpan is a major undertaking, and many swappers are not interested in hacking up their car. The alternative is to use a forward-mount adapter with an aftermarket crossmember. This allows the T56 to fit with minimal tunnel modifications.
The key to correct transmission angles is to position the tailshaft at an angle between 2 and 5 degrees, and it should match the rear pinion upward angle. Too little or too much causes vibrations and premature joint failure. An angle finder should be used to set the transmission at the proper angle. The U-joints must be “working” to last; if the joints are positioned at 0 degrees, they will burn up.
For late-model automatics, you need to carefully consider the same factors. Here, the main issue is that they have bolt-on bellhousings, and that creates a clearance problem in the front of the transmission tunnel. The stock setback pushes the transmission back enough so that the bolt flange hits the tunnel. Serious modification is required to rectify the issue. Moving the engine forward 1 inch solves the problem, and the factory crossmember can be used as well.
When installing an LS engine, getting the driveline angle correct is critical in terms of strength and reliability. The transmission must be angled between 1 and 5 degrees downward on the yoke. For performance applications, 2 degrees is optimal. An angle finder (available at most hardware stores) can determine this angle. You place it against the tailshaft and let the needle rest until it points to the drive angle. If the stock crossmember bolts to the engine and the drive angle is between 1 and 5 degrees, it will work.
For an LT1 swap, the factory crossmember sits way too high, keeping the tailshaft from sitting at the correct angle. You could modify the crossmember or replace it with an aftermarket version. Lowering the factory crossmember requires dropping the center section because the exhaust doesn’t clear if the entire unit is lowered. The transmission needs to drop about 3 inches.
If the drive angle is not between 1 and 5 degrees, the crossmember must be modified so the driveshaft has an adequate angle. Several methods can solve this problem, but it depends on the crossmember you are using. Two versions of crossmembers for A-Body cars are available: a tubular unit for open-frame cars and a formed steel unit for boxed-frame cars (all convertibles, El Caminos, most GTOs and Stage 1 Buicks, and some Oldsmobiles).
With late-model transmissions, the most common issue is the tailshaft sitting too high in the car. Because the crossmember sits on top of the frame, it cannot be lowered easily. You can cut and weld the ends to lower the entire crossmember or you can drop the center; either one achieves the same end. The crossmember, however, also has raised sections for the exhaust, so lowering the center is the better option.
If that’s not possible, a new crossmember is required. Numerous aftermarket crossmembers are available for open-channel frames. Some motor mount adapter brands, such as Hooker (Holley) and BRP Hot Rods, are complete systems, designed to work with the same brand’s transmission crossmember. If you have a boxed frame, the open-frame transmission crossmembers don’t work and you need a special crossmember. G Force Performance makes a heavy-duty crossmember that fits the A-Body boxed frame quite well and is perfect for LS swaps. BRP Hot Rods makes a boxed frame crossmember designed to work with its swap kit.
The keys here are driveline angles and keeping the tailshaft square between the frame rails. When fabricating crossmembers to support the transmission, use materials that are strong enough to hold the weight and torque of the transmission. Tubing (round or square) is a good material to use because it provides structural stability with less overall material thickness and weight. Flat-plate steel requires thicker material to achieve the same structural integrity. Angle steel is another excellent material for custom transmission crossmembers.
For the Buick, I opted for a G Force Performance Products crossmember because it’s the only one available for boxed frames. To mount it, you drill out the stock bolt holes to 1/2 inch.
The G Force crossmember has a dropped-center mounting plate, so the transmission sits lower in the chassis for tunnel clearance, about 3 inches. You can always adjust the transmission mount with the provided mount spacers. I used one spacer on this install.
Placing the nuts into the chassis can be difficult; this trick helps you install them: Roll half a thread onto a long bolt and then thread the nuts onto the upper bolts. These nylock nuts are a little easier to work with than other nuts.
The beefy crossmember has large hoops for exhaust clearance and fits the chassis quite well. It’s very strong and provides excellent support and strength, which helps maintain chassis strength. It is a little heavy.
I also installed the G Force sliding transmission mount so I had 2 inches of travel to mount the transmission to the crossmember. When sliding motor mounts have been installed, a sliding transmission gives you extra flexibility so you can correctly position the engine, transmission, and driveshaft for the best performance possible. A sliding transmission mount is especially helpful with sliding motor mounts because you can quickly exceed the reach of the factory slots in the crossmember.
I installed the mount and then slid the crossmember in place. You may need to remove the sliding section of the mount and roll the bolts into the crossmember slots because the bolts are actually studs and are made long for installs that need spacers.
Performance Project: Choosing a Driveline