Good to hear you did not make the critical mistake a lot of people make when building a turbo...to quote those who cannot pronounce the letter "H"...It's gonna be UuGE! Good luck with your build. If you need any help with the carb setup, give Bryce at Turbo Performance a call. He has done many of them over the years. I also believe it would be wise to spend your money on an EFI sled. Tuning is way more predictable and easy to do.
But, if it turns out that the 800 does not make enough power, build a turbo 4 stroke. I got a little out of control building my chute climber 2 years ago...more like a lot. Any way, I prefer it over a turbo two stroke for climbing the big stuff. It also has a whole lot more bottom end grunt! Turbo'd they build way more torque and generate crazy track speeds. In addition, as was previously mentioned, they can handle a lot more boost.
In building SnowFlake (my turbo Viper), I learned a lot about what it takes to really build a high horsepower chute climber. Custom parts are ridiculously expensive. You have to know when to stop (read this sentence again). No matter what your ego says, you cannot really ride a sled that makes big power (300 hp and up). Otherwise, you ride around at half throttle all the time scared of what will happen to your life if you unleash the atom bomb. My fully built race motor is capable of more than 400 hp at elevation in the 30 to 35 pounds of boost range. Trust me, a sled that kind of power is not rideable on the mountain. My advice, keep your build reasonable and have fun.
Finally, you will likely find it difficult to keep your front end down once you get your sled dialed in. If so, call Kevin at KMOD. He builds a quality suspension that will put all of your new found power to the ground. Stock suspensions really cannot handle the kind of power transfer turbo sleds generate. Good luck!