Safe at highway speeds can be achived with new bushings and other wear parts on the stock suspension, and of course making sure the brakes are in great shape.
The guy down the street has a TBI 350 and a TH 400 in his. His mileage will probably be better than mine with a Cadillac 500 and TH 400, but still not spectacular.
I've seen a Cummins in a 1-ton but I don't know that it fit all that well. The truck over in Roy has a 6-cylinder Cummins. No idea what that cost and I'm just not a Diesel guy so I didn't ask any questions.
I don't think the stock suspension would be all that scary at speed even in corners, because it is so stiff. Its when you take leafs out to ease the ride that you loose roll stiffness.
Lots of things to think about...
