McGowen rebarreled a Mark X to .376 Steyr for me a couple years ago. I've not been able to try a lot of different loads as of yet but so far so good. I like your idea of a Bushmaster bolt action and have that on my "To Do" list as well. Have no experience with them but these guys specialize in .450 bolt guns:

http://bohmanarms.com/thumper/build-a-thumper/