OOOF. the trans van i looked at this weekend had the frame rotted out from the rear axle back, including the shock mounts. not willing to take on THAT much project - i'm not convinced that it could be saved without some significant welding, welding that i'm not capable of given the critical nature.
i have one more i can take a look at in this area, and then that plan may pivot to a bus or box truck.
to that end, i may be going to look at a 17'ft chevy box truck with a 6.5L diesel in it. forums that discuss that motor say that it regularly is in the mid to high teens when conservatively driven - with the city performance gearing in place, so, there may be some opportunity there.
a good buddy from highschool also indicated that he has a compound turbo setup for that era motor and, from what i'm reading about those, that could be an improvement possibly if properly set up with more highway oriented gearing.
has me rubbing my chin anyhow