What you plan to use the bus for will play a major role in which engine to use. If you just plan to putt around locally and not tow anything the 383 perhaps with a turbo 400 tranny should work out OK. On the other hand if you plan on highway driving and some towing, a small turbo diesel would be a better bet. Both Tango and I have Chevy buses that share(d) the same drive line as yours and we both have opted for diesel (Tango has a Cummins and I have an Isuzu). I've driven my '35 more than 10,000 miles, often with a 2700 pound toad and the extra torque from the diesel has come in handy. Furthermore, the fuel economy with the diesel will be better than with gas. My little TD averages 17 mpg on the highway where most of my driving is done. Whether you choose gas or diesel, if your bus has a two speed rear axle and you chose an automatic trans you might consider changing out the rear axle to a single ratio suited to your engine and trans. Even if you just change out the motor consider a brake upgrade. The original brakes on these oldies were poor at best. While they were probably OK at 45 mph on rural roads they will be nearly worthless at today's highway speeds. Jack
P.S. Don't forget to share some pictures of your new toy!