Some days it has a BIG effect. I've seen density/altitude in the 10,000 ft. range during the summer.

The last engine I built for Denver had 12.4 to 1 mech. compression and good quench. It was a 383/451 stroker with a cam in the 260º / 266º range. For fuel I used a 4 to 1 ratio of 101 race to 91 pump. just to be safe. I was shooting for 180 psi theoretical cranking compression. I got my mechanical compression near that and juggled duration slightly. Getting your cylinder pressure up and carefully choosing parts is the name of the game out there.

Give Greely Auto Machine a call and see what they tell you. You'd be surprised how many people that have built engines out there don't know to ad 1 point for every 2500 ft. of elevation. When the engine works out to be a dog, they just blame it all on the altitude.