At higher altitudes with the thinner air you need to get more into the cylinder-----that means bigger carb, higher flowing heads/manifold and a carb that has been optimized to get the fuel into the airstream-----now you need to have cam timing that is shorter to stop reversion but with more lift so that when valves are open the mixture flows at the best rate for the absolute pressure available . You need the most open exhaust that you ( or the law ) can stand the noise from- lots of split in the overlap so exhaust gets out of the cylinder before intake opens and then open intake at highest rate toward the best flow numbers. Dist needs to be curved and vacume advance doctored for more control ( those old Ford dual diaphram dist work good)

There is a modern type control ( think Edelbrock sold it for there carb manifold on LS type engines?) where you can program the spark even remotely to drivers position ( left side dash?)

If you are building an engine from scratch for high altitude use consider the rod length ratio to stroke for more optimun piston speed at the top and bottom dc.