Get your scale out, measure the firewall to rear of radiator distance, that is how much room you have, get rear of block to front of cooling fan distance to determine which engine will fit.

Ed

 

You probably can fit one. Some of the delivery vans came with the 6bt, but not the P30. Will probably change things like driveshaft length, transmission placement, etc. May need to rebuild the dog box area since the engine is 2 cylinders longer. Don't know if you do any road speed traveling, but an OD gear in the transmission would be a plus. Unless there is sentimental value, you might find a 6bt van cheaper.

 

Memories from the past:
https://www.4btswaps.com/forum/showthread.php?6874-will-a-6bt-fit-in-a-stepvan

Yes it can be done. But have alot of patience when doing it. You will have to make everything. From all the lines to running all new ones as well. I made all my lines from black iron pipe and threaed it all myself. I made new brackets for 4 remote filters. I can't stress enough how everthing must be made to fit. If your a weekend warrior this project is not for you. It will take alot of skill and alot of fab work to get this thing right so when you drive it things aren't falling off of it or things are held in by duck tape.These P 30 series vans were never ment for the allison cummins engine. You will have to get a custom made driveshaft 5 inch yokes. Your old drive shaft out of it was a 2 piece and near 10 foot long your new one will be 4.5 feet long and 4 to 5 inches around. You can't throw the thin drive shafts that came out of it back into it and be expected to pull large amount of weight and run the big allison trans. It won't work cause the yoke won't fit it and the new added stress to it will twist it like a pretzel. If you guys want anymore info on it just PM me and i will shoot you back asap.
Thanks Chris

http://s411.photobucket.com/user/Cumminsallison/library/?sort=2&start=0&page=1