I had 7 SD1s, can't remember if it was 2 or 3 out of that were straight 6s, rest were V8 including the first 2 that used the same engine and box.
These days, the V8 in a Transit is usually the 1UZ Toyota/Lexus 4.0 V8 although some have used the VVTi version of the same (3UZ i think it is) and the Lexus V8 is becoming more widely used as a replacement in Volvo 2/7/9xx when a bigger engine is wanted.
The Toyota/Lexus A341(E) autobox is pretty much the same as the AW-71 box fitted to most of those cars to start with and even if they didn't have an AW, because similar models had them, the mounting brackets etc exist as do the gear lever linkages etc.
I keep having moments of temptation to create what my 760 should have been originally! When they developed the PRV V6, it was originally destined to be a 3.5 V8 then the early 70s fuel crisis hit and they chopped 2 cylinders off it.
With the improvements (including longer stroke/larger bore) Volvo made about 1987, it's now 2849cc so rounding that up to a V8 makes about 3900cc give or take.
The other option is a Rover SD1 Vitesse engine that's been taken to 3.9 as that would be closer to what the original would have worked out as.
Nah, 1UZs are more plentiful!