So apples to apples 603A or a 606A obviously the 603A would be easier as the 606 would need a FI pump off a 603A to avoid the electrics. I like the durability of the 606a better.
But then again the later 603s had the head issues addressed. And as long as I avoid the rod bending 350 I should be good. Have you had any issues with your 350? I was thinking a 300d 603A with the 350 exhaust and intake manifolds as well as the 350's pump would make for a pretty nice engine.

My goal. I really want the best long term durable diesel (like a 617A) not necessarily a tire burner. I will be doing all the work.

Thoughts
Mark