OK , just so ya know , I'm not with the DVKK but I'll try and answer anyway.
A 78 bus should have an 091(76-79) bus tranny and a 2 litre bus (type 4) fuel injected motor. No reduction gears here. I believe it was 1976 when they changed the bell housing, flywheel, starter combo. keep those 3 pieces together, either early or late and you should be able to mix and match any type 4 motor and IRS bus tranny. You could even put a beetle motor in it with an early bell housing and some costom mounts etc. Ofcourse I'm over simplifying it all but I think thats kind of right. Someone please correct me if I'm wrong or not making sence.
