Motor mounts and trans mounts can be easily fabbed, and even driveshafts built if ya phase them correct and check the balance. I have found that it often takes as much time to mount oil coolers and/or build a new core support for a larger radiator, than making mounts.
Also the exhaust pipes have to be mig welded, fuel lines run, etc.
Changing a diesel engine i think, is easier than putting in a modern drive train in an old car.
I am not an expert on that either, since it was like 1966 when i built my first car frame, etc for the drags.
ol Trunt is the expert...