Swapping Turbo 400 in 1980 Stepside

[TexasStepside]

Question - I have swapped a 454 into my 1980 Stepside that was originally a 250/TH350 combo - I retained the TH350 just to get the truck mobile. I want to swap in the TH400 that came with the donor truck (1978 1/2 ton, LWB) but I noticed that the carrier (center) bearing for the two piece driveshaft from the donor truck mounts to a crossmember that also doubles as the rear cab mounts - my Stepside does not have this crossmember, but rather just the mounts for the rear cab. I am planning to transfer this donor truck crossmember and shorten the piece of the driveshaft from the carrier bearing to the rear axle. I am assuming that this is the way the factory setup is with a TH400 in a SWB truck. Any information, pictures of this would be appreciated.