1971 3/4 Ton Transmission Fitment Issues

[1961ThreeQuarterTon]

I have a 1961 3/4 ton GMC, 4 speed, hydraulic clutch. I bought for the Hot Rod Dirt Drags.* It's pretty rough, and has been built and rebuilt several times before I bought it.* I don't know what is stock or fabricated on it.* I bought an 1984 one ton Chevy van from a buddy.* It has a built 350 v8 with a 400 turbo.* My intentions were to put the package in together.* After pulling the I6 out of the truck, and then the motor and trans from the van.* It was very obvious there were length issues.* The trans was going to be way too long, and the tail shaft would not fit because of the rear crossmember that supports the torsion arm system.* I did not have time to do allot of fitment as I had to get it to the Dirt Drags.* It looked as if the motor mounts had been changed earlier, and probably not in it's stock location.* Anyway I was able to swap the flex plate for the friction plate on the 350, and shoehorn the 350 in.* It wasn't tight on the sides, but there was almost no room in the rear for the distributor against the firewall.* It worked though and I was able to get it to the Dirt Drags.* We had a blast, and I decided I want to continue to fix it, and either get the 400 turbo to fit, or a m21 or m22 Muncie.* Ether way I am sure the tail shafts are going to hit the rear crossmember.* The question is.*

Has anyone else ever had this issue?*

Is the motor in the wrong location?*

Is there a replacement crossmember that will handle the torsion arms?*

What has anyone else done?

Surely I can't be the only one to do this, as I looked through lots of threads and didn't find anything.

All help is appreciated.