Lean it out an advance the timing just a couple degrees. The rule is supposedly 1 degree for every thousand feet roughly. I also bought a 600 Edelbrock and it ran great out of the box and I'm at 5000 feet give or take.
Me to, about 5000.
Ya need to drop the primary jet size down about 2-3 sizes, then re tune your idle mixture leaner, and advance the timing. (After you are there of course)
I used to drive from Susanville to Reno to Stockton, so I had the opposite problem.:D