I've heard guys on here say that RPM with no load will cause the carb to go lean, thus red headers.

That said, before I knew anything about AFRs, rich/lean, etc. When I took my car out to Colorado, I followed the chart that Edelbrock provided with the carb, on what jets/rods to use at various elevations. The car ran and drove fine the 2 years I had it out there. ALthough, it didn't have any power, but what NA motor does at 7,000 feet ACTUAL elevation? I'd bet the DA on most days was up near 10-12k