In my area we don't have to worry about rust. But I think you are right about there being very few late model manuals for sale. Even if I settle for a 5 speed.
I had the 351m but up graded it to the 400m and that was a big difference. It ran great at that altitude because of the 2 barrel carburetor. I would like to stay away from the 4 barrel carburetors. They fall flat on their face out of a hairpin turn from almost a dead stop at that altitude. Nothing gets good mileage at that altitude but 4 barrels are sick.
The newer apx. 350 cu inch engines would likely have the power. But I don't think I'd find any with manual in a latter model.
That's why I was thinking of the Ford v10. I forgot about the 8.1. So few of the Ram v10's were made I bet it would be an impossible task looking for a manual one. I think they all had port injection early on.
My four speed is fine. I just need the a overdrive if I decide to travel farther, more often.
I may have to start banking money for the new Ram 6 speed. I just need to push myself away from the slots more often. :>)