Re: Question about my 721 action
I have one for a .270 WIN that my grandfather built in the 1950s. It is the precursor to the R700. Getting original parts for the 721 is challenging. I had an issue with the extractor and I ended up needing to get a sako style installed since parts weren't obtainable.
In terms of being "desirable," I cannot really help you there because I don't deal with antiques. Performance wise, it's probably comparable to an off the shelf R700.
The 338 RUM is a new cartridge, released in 2002 per
http://en.wikipedia.org/wiki/.338_Remington_Ultra_Magnum and "operates at very high pressures." I would call the gunsmith you plan using and getting his opinion. For some more perspective, you could read
http://www.bordenrifles.com/338_lapua.pdf According to this, Borden's standard actions can accommodate a .338 RUM. In any event, assuming a 338 RUM fits in your action, you'll need a magnum boltface bolt, which means you'll probably need to order one from PTG and have it fitted.
Perhaps it might be more cost effective to leave the 721 as a hunting rifle, sell it for a few hundred dollars, and use the funds to get a stiller TAC 338? That's what I would do anyway.