[justcuz]

Are you going to keep those front springs? I see they have wear buttons on the ends, but thickness of the material and the fact that with 3 leafs the weight of the engine did not sag it much, means those springs are pretty stiff. (whew, run on sentence) It seems to me that a lot of flex/force would be transferred to the chassis with those springs. Have you seen the conversion were they use Chevy 1/2 ton 52 inch back springs on the front? They move the front mount forward about 2 inches. This would give you a little more clearence for the Dana 60 and allow more spring flex than your lift springs would allow. It should save the chassis. The only reason I bring this up is I had a lift on my 1976 3/4 ton that only had 2 leafs. I swapped them out for another companies multi leaf spring with thinner leaf material and the ride was improved dramatically.
How much lighter is the 4 cylinder Cummins? I have heard they weigh about 800 lbs, which is not too much heavier than a complete big block, correct?
I enjoy reading this thread because you have been very creative in making things work. This is a credit to your skills and thought process as a mechanic and fabricator.