My dad has the 2900 R (1974) and quite possibly I will be inheriting it someday.

We have the 454/4L80e in it and got 8 MPG with the TH400 and yet to get enough driving data with the 4L80e.

I also would like to consider the LMM/Allison 6 speed. Weight on the engine is comparable. 1000 should be similar to the 545.

Jim Black told me that vibration was a problem with diesels in the 2900R and it would need significant body (structural) strengthening that he does. the Duramax may not have those vibration issues (that would be my hope).

I am impressed with the 454, and possibly will keep it in favor of diesel... But may inject it to improve efficiency