The legalities of assembling the rifle are beyond this particular forum.
Hunting with a .50 is (AFAIK) legal everywhere. Somebody out west introduced a bill trying to outlaw the .50 for elk.
IMHO, it's sort of a waste. My understanding is that moose are fairly close range targets. If your going to use a .30 bullet, you might be ahead dollar wise getting a good rifle. Additionally, the saboted bullet should be selected with care as the velocity may over stress the bullet. (4000 fps?) I would think hard about a Barnes X bullet myself.