After looking into it, the measurements of the .458 Winchester magnum look very appealing. It looks like mag well length and bolt face would both work. Am I reading this right?
Barrels may still be available from Gun Parts Corp (Numrich Arms). Haven't looked recently but they had 458 Win Mag factory barrels for No. 1s relatively recently.
TS M-77 is a straight conversion from the 7mm RM to 458 Win Mag just with the rebarrel, though you may have to slightly tune the feed ramp for the larger diameter bullets especially if you plan to use flat point solids. For your purposes, many 0.458" diameter round nose and rounded HP bullets are available so they may feed with the factory ramp profile.
The 375 Ruger idea is a good one, though I'm not sure how well they would feed through the PF TS M-77. You would need to see if the TS follower works or if the 375 Ruger Hawkeye follower would be needed. Again making up some dummies and trying feeding would answer that question. I can try it in my 458 TS M-77 and let you know how well that works as I have some dummies already made up, if you consider that route.
You could make up a few dummies with the bullets you intend to use and try feeding up the ramp as is with the long TS factory magazine follower in place.
BTW, if you get a tapered expander plug (a couple of diameters in steps) you can even form your 7mm RM cases to straight wall 458 Win Mags as the latter are hard to find right now. 300 Win Mag cases also work as a starting point. I anneal the smaller caliber cases before expanding to 458 Win Mag.
