After mounting the engine the next step was to build the transmission crossmember. The first step in that process was cutting off the factory mounts and grinding the framerails smooth. I ended up putting my crossmember about 6" behind the original one and didn't want to try some sort of cantilever arrangement so they weren't useful to me. Next up is getting the centerline of the transfer case input shaft in the same position as the engine, 1" offset to the driver's side. This is a little harder of a measurement to set up because the frame rails are not parallel in this area. The rails are angling in towards the center so you can't just toss a square on them.
I first hung a plumb bob off the input shaft to mark the centerline. I then used a tap measure to check distance to the frame rails and get things close to position. After that, I set up a straight section of aluminum spanning the two frame rails and measured the distance from the aluminum to the left and right most edges of the tcase flange, since the tcase was close to position and therefore perpendicular to crank centerline this got me roughly parallel. After that I checked the aluminum to some major features such as the front spring eyes and axle, made a few tweaks and called it good. Then, using the plumb bob as my crank centerline I measured the distance to each frame rail and set it to the 1" offset mentioned above.
In the pic below you can see the measurement setup and the beginnings of the trans crossmember. I had recently purchased a TIG welder so building the crossmember was a chance to practice some, I chose to make it from 3/16 x 3" steel flat bar. The adapter has a mounting pad on the bottom that fits the standard Chevy TH350 trans mounts, I chose to use the Energy Suspension urethane mount p/n 3.1108G which I found at my local parts store. I didn't want to cantilever mounts down below the frame so I bent the flat stock to follow the bottom of the tcase and then come up to meet the frame rail. There's a notch at the very bottom of the tcase to clear the bolts as you'll see in later pics.
With the bottom of the crossmember decided it was then time to create the rest. I chose a box design to maximize the strength, later I plan to add a skid plate to the underbody and this will be the main rear mounting point. The shot below shows all the pieces after cutting them out and prior to weld. All this was cut from the 3" flat bar, next time I'll draw it up in CAD and send it to the waterjet shop, this took a long time on the band saw! The bends are done in a press brake I have, as you saw in the pic above it's a pretty close fit to the transfer case so laying this out took some time and careful measurements.
After cleaning mill scale and prepping the surfaces I fired up the welder. This is an in-process shot to show some of the other details. The mounts to the frame rails are simple plates that get welded on parallel to the crossmember. A 1/2" bolt carries the load so inside the crossmember I welded in sleeves made from 3/4" OD .125" wall tubing. I also welded in two nuts on the bottom face to act as mounting points for the future skid plate. In order to provide maximum ground clearance I elected not to box it all the way across so that the tcase can recess into a U channel at it's lowest point. The top cover plates dive into the U-channel and are welded at the bottom, leaving an open area where the urethane mount nests into.
I swear I had a pic of it fully welded on the bench but can't seem to find it. Unfortunately it's in use now bolted in to the truck so it's hard to show that last bit about how it fits around the mount. Here's a final shot showing it installed in the truck though. The fit to the tcase is tight with around 1/4" in the closest spots but given the stiffness of the urethane mount I'm not worried about it hitting.
The last real question is exhaust routing. On the driver's side there's quite a bit of clearance and I'll route the pipe over the top of the crossmember. On the passenger's side I played around with the mounting height quite a bit in the mockup phase to try and find the best compromise. The brake and fuel lines run up the frame rail here so it's not a great place to be routing a hot exhaust pipe. The clearance between the tcase, the rail, and the underside of the body may be just enough to snake a 2.5" pipe through there and with heat shields to protect the lines and floor it might work. The alternative is to go under the crossmember which will leave the pipe hanging even or maybe just a bit below the bottom surface. With a good skid plate design, that might be okay too. I'm not yet to the exhaust fab so that challenge will be solved later.
