[Little]

A swap to an inline 6 is a big deal, involving rad relocation, replacing or blending wiring harness etc. As others have said the eco/.ecu and related wiring would be a killer for me. I have done many Diesel engine swaps and have never been able to get the wiring myself, so be sure to have this figured out before starting project.

I would install jakes and valve covers on the used engine from junkyard and go. I did this with our MC8, swapping in a 6V92 for 8V71. That was 4 yr ago and still humming along.

You have a 40 yr old classic vehicle. How much are you going to drive 75 mph??