I get everything your saying, except what you didn't say. What is "much power"? The op is looking for a driver, not a 12 second car. I agree, I would cut the heads in that altitude. However, I think the op will be happy for what he is expecting.
Oh, by the way, I did a throw together 318 for my brothers Demon in Loveland, Colo (about the same altitude) a few years ago. I did the MP .474 cam, cut .040 360 heads that were home ported, and 4bbl with hedman headers. Short block was never taken apart. He out ran a mid - nineties corvette (4 exhaust pipes in the center), along with some other stuff. He had 3.55's and an 11" TCI breakaway converter. All used parts, including the 318 2bbl we started with. He was happy for the few cents he threw into it. I think the op will be happy also.