[MrE]

Keith Frerichs (@retro) has repowered 2 70's era Newells with mechanical Cummins 8.3s and Allison 6 speed transmissions.
https://newellgurus.com/showthread.php?t...ht=repower

Scott (@wepaddle4) has also repowered his 1983 with a Cummins 8.3 mechanical.
https://newellgurus.com/showthread.php?t...ht=repower
https://newellgurus.com/showthread.php?t...ht=cummins

Send Keith a PM and he'll let you know all about it. Pretty sure his advice will be to buy a donor vehicle instead of an engine and then all the pieces to make it work.

Quote from Keith after doing the repower; "I highly recommend finding a donor bus with a good engine and trans. The side rad is also a good thing.
This way you will have most of the parts you need, even extra parts like batteries and tires etc. If you try and buy everything separately it will nickle and dime you to death. The labour took us about 100hrs so if you are going to pay someone to do this it could get pricey if its at shop rates these days.
I bought my donor bus for 5500. Too bad the engine wasnt as good as I thought but these things happen. I am happy the parts for the 8.3 are cheap. 903 parts are harder to come by and expensive, also still alot of hours labour so the cost would be probably close to the same as doing an upgrade but in my opinion you will have a much better engine and trans with more readily available parts if ever need be. Plus it will be more sellable with the newer power. I was getting over 9mpg pulling my 22ft boat last year fully loaded. Even worn out it was still hands down better than the old 555. Hope this helps!!"
https://newellgurus.com/showthread.php?t...ht=cummins