I think you are going to want to look at getting a 4 speed stick over the C6. With no lock up on the torque converter you will spin slightly higher than the stick. With the stick you get a deep first to start the load down the road with so you can run a higher rear gear for better cruse MPGs than the auto. Also it would be easier to swap in a ZF-5 then you get a deep first AND over drive.
The diesels are pretty indestructible if you keep them cool, oil changed, and coolant additives in the 7.3L (look up cavitation on Wiki to see the reason for the additives), the 6.9 is know for head gasket failure (some have never had this issue others have had it), this is pretty much the nickle crash course on old ford diesels.
I love old Ford junk too. I have two inline six's and a 7.3L IDI tow pig.