At your altitude you might have more fun with a turbo diesel than a gasser, at least a normally aspirated one.
Really it depends on your budget and circumstances. If you use it to haul and work and occasionally take it to the track you will probably be happiest with a diesel swap. If your budget is tight go for a 12-valve Cummins and if you have more play money you can go for a CR Cummins or a Duramax. If it is a dedicated race rig, I'd actually consider selling the truck for what you can and buying a chassis that is more drag race friendly then put money into that rather than spend a ton on the big heavy truck to go kinda fast...
Either way you go, keep having fun with it... Don't bite off more than you can chew and end up with a truck sitting in storage while the project is stalled due to lack of interest or lack of funds... Believe me, you'll kick yourself if you had a fun vehicle and let it sit for years as I have with my Camaro!!!!:doh: