I can drive 80 mph all day long if I'd like. Much more comfy at 65. The radials and the O.D. are the magic. Other than headers, dual exhaust and HEI, my 235 is stock. It's still a little weak on the really steep hills and sure it would be worse at your altitude. I'm currently having built a 261 stroker that will be significantly stronger than my motor. I'm doing it because I can retain the stock "look" of my original motor, it's a bolt in, and I REALLY like the look up a hopped up 6. I could have bolted in a new fuel injected 350 crate motor for less than 1/2 the price of what I'm doing.
Even a stock 292 would be a good work motor for your truck. Hop one up a little and get an auxillary tranny or a 700R and you have the best of all worlds. You really gotta consider upgrading the gearing, tires, braking, steering, and suspension if you want it to perform like a modern truck. The engine should be the last thing that you do. Just my 2 cents.
