Non Chevy engines in Chevy trucks.

[pontiac6269]

I have a 81 3/4 ton 2 w/d that I'm putting a Pontiac 400 in. The 350 has a main bearing rattle. Are 3/4 ton mounts in the same position as a 1/2 ton? I had to use the lower mounting holes on the mount plates, also had to move the plates back 1 inch on the drivers side and 1 1/2 inches on the passenger side. Then drill holes in the frame and crossmember. I've read where others have done this swap in 1/2 tons and just had to move the frame mounts up to the upper holes. I'm using the original turbo 400 with a TCI adapter plate so the trans is mounted in the original location. Has anyone else done any kind of swaps like this or other engine familys?

Thanks,
Eric