Bob,
I've got a 52 with an early 70's 250-6 cylinder, stock tranny, and rear-end. It's very reliable, and will run down the road very comfortably between 55-60 mph.
I also have a bone stock 46, with a 216. It's not given me a problem, runs like a sewing machine. Down the road, it will run pretty comfortable between 45-50 mph.
I think you really need to determine what you'll be doing with the truck, and how far you think you might want to venture from home prior to making any decision.
I'm partial to keeping the sixes going. Especially if you're going to upgrade the rear gears and throw some other goodies in it.
The V-8 might be nice, but come cruise night, or car show, you'll probably be the rare one if you keep a 6 and split the manifold.
