I have seen many posts about people having this issue. I have not written a post because I didn’t have pictures until now. I did this simple mod 11 years ago on my daily driver. Since I was under my car this weekend installing a new aluminum transmission pan and temp sensor, I figured I would finally take some pictures. I made it from 1/4 inch plate steel. The attachment for the torque arm has several holes so I can easily change the pinion angle, although I never have.

 

The problem with that kind of setup is, it applies a torque to the original crossmember. Instead of a load in line with the bolts, straight up and down, the force is applied way out at the end of that plate, and it tries to twist the factory one right off of the "frame". The "frame" is already bad enough about cracking around those bolts; over any kind of long term or high mileage, something about that long lever arm would almost certainly fail, sooner or later. T-56 crossmembers do a similar thing. I've seen cracking in my car's "frame" there.

One of those things that's possibly OK in a static situation, but dynamically, it's problematic.

For a 350, much better to simply add the 2 front bolt holes that 82 had, and fab up something that mounts directly up there.

I do like your pinion angle / instant center thing though. As long as you can set it to where the pinion angle at an acceptable spot, at the same place that it the instant center needs to be, it should work pretty good.

 

I agree that it could be a problem in a high horsepower application. That's why I keep my power level nice and low.

Maybe I just got lucky, but, it's been working fine for 11 years.

 

I wasn't really thinking so much of high horsepower; more, just driving on bumpy roads alot.

I'm glad it's working. Best of luck to you, that it continues to do so. It looks well thought out except for that one detail.

 

I was looking at the mount and wondering that also. Is there front to rear movement of the torque arm, or more just up and down?

Nice fab work OP.

 

Don't Spohn and the other Suspension Companies produce a THM350-Swap Transmission Cross-Member?

 

There is a very small amount of front-rear motion, therefore the stock rubber slider thing. Probably no more than a half-inch or so though. Most of the forces are up-down.

 

The Stock Torque-Arm definitely moves forwards and backwards inside of the Stock Mount.
However the amount of movement is very small... around 7/16" in each direction.

As the Rear Differential/ Rear Suspension compresses and extends...
the Torque-Arm must be able to move with the Rear-End (back and forth in the Mount).