The 721 is a great action, and would be an awesome action for a .280 AI or .25-06 AI build down the line (hint, hint
)...
Also, to answer your question, the 721 WILL fit in a 700 stock, but will require some trimming and fitting for it to fit properly, as it is slightly different than the 700.
But for a custom tactical/long range .300WM build, I think you would be better off to put the 721 in the safe for now, and buy you a cheap used 700 magnum (7mmRM or 300WM), and start from there. Especially since parts are so easily and readily available for the 700. You should be able to find a cheap used ADL or BDL at a pawn shop for $300-400 and start from there. That's what I do instead of buying new bare actions, I go for the cheap used ADL option, because I can find them around here for $300-350 out the door, and I can still turn around and disassemble the action and sell the old parts for about $50-100, and be money-ahead. You do this, and you're at $200 for the action, then $150 for a new ADL-to-BDL conversion from Brownells, $125 for a Timney trigger, $45 for a stainless PTG .250" tapered lug, and then the rest is stock, barrel blank, and smithing fees. You can build a custom 700 for under a $1000 (not including optics).