I'm doing my first tune up on my Dart since I moved from about 500 ft. above sea level to my current 2600 ft. above sea level. Would this change what I should select for a heat range? My motor is a 364 ci SB with ported 308 iron heads, about 9.75:1 compression, full length headers and Comp Cams hydraulic cam, and MSD 6AL with Blaster coil. I've been running Autolites or NGKs gapped at .050. Anybody have any specific suggestions?

No, but you do need to address the carb jetting and possibly the timing.

Nope, will probally induce spark knock..

Think about the spark plug and what it does at sea level and again at the higher altitudes I wouldn't be afraid to try another step hotter if your running a cold plug before, say a RN6YC compared to a RN9YC or a Autolite #72 or 73 switching to a #74 or a 74 I've done just the opposite of what your doing, ran the same motor at higher altitudes and then took it lower, one heat range colder made the differences of the motor pinging or not at the lower altitudes How do the plugs look now compared to what they looked like at the lower place?

As said with the thinner air drastically lowering the density which is going to change cyl psi/pinging point I would retune the dist/carb. Get the carb reasonably close (A/F ratios) then fine tune the timing: initial/same total (if correct now)/springs/vac adv. then fine tune the carb a bit more as needed. (1) staying just under the pinging point (2) correct or near correct A/F ratios