I'm much closer to sea level here in Georgia. You are correct, altitude plays a factor because the air is less dense. There are a number of ways to compensate for this (higher nitro content, bigger engine, lightening up the plane, etc). Although I lived in Albuquerque, NM for a number of years, I did not fly R/C then so I have no personal experience. However, many people on these forums do and they can offer you some excellent advice. You could probably do a search on "altitude", among other things to find answers. Hopefully someone will chime in here to give you a hand to help you understand. I'll just end up confusing you. Like others have pointed out, there is a tradeoff when you add weight to a plane (by going with a bigger engine, for example). You increase the wing loading for one, which changes the flight characteristics of the plane. If it flies the way you like it to fly, it doesn't matter. That is the most important thing. In my opinion, you'd be better off using a 46 size engine, along with keeping the plane as light as possible. The 75 might work just fine, you'll have to try and see. I'm all about having too much power and managing the throttle to keep things from getting out of hand, but too much wing loading can be a problem. I live in Clovis New Mexico altitude is abou 4300ft. At first i would always try to power my planes with the biggest highest power engine I could find. Vertical was great but because of the heavier engine/higher wing loading, flight performance sucked.
The biggest and most important thing to consider when doing 3d is wing loading then power. In my opinion it is best to choose an engine with the best power to weight ratio. I have more fun with an airplane that is moderately powered with a low wing loading than a way overpowered one with a heavy wing loading. Not to mention it's much easier to fly.
At higher altitude engine selection becomes more critical and extra emphasis on building planes light.
I think Gmoneypit was giving good advice.