3.9L Cummins/Isuzu diesel swap in 89 Suburban

[87Skier]

I've got an 89 Suburban (picked up as a project vehicle, just unsure of the project) that has a motor that runs but I suspect will have bottom end issues in the next couple years. Sounds like a rod knock on startup, but could be anything. Already has top end issues.

Moving passed that, I thought this would make a good candidate for a diesel swap. I know the 5.9L Cummins are incredibly long and hard to fit in any vehicle and since this came underpowered from the factory, I thought putting the smaller diesel in would be a good compromise. I've been looking at parts and so far it isn't adding up.

V2500 Suburban (Square body style)
350 TBI engine
Turbo 400 (3 speed trans)
NP231 (chain driven, right hand drop) transfer case
4.10:1 front and rear gearing
235/85r16 tires

Currently at 60mph, the engine is at 2600 rpm. I am shooting for around 1500 rpm at 60mph. Even if I swapped in an OD transmission that still puts the engine close to 1850 rpm. I think it is still on the high side.

What are your thoughts on this? Which transmission should be used? 700r4? I've heard that 4bt's are a lot more expensive but could save you money in the long run since Isuzu parts are more expensive.

I would like to avoid changing out the gearing if possible.