Rocky, Brownells and Lothar Walther list barrels for small ring Mausers, I would stick with the 7 x 57. It is a fine cartridge, very accurate, can operate well at 45,000 psi, and will feed flawlessly in your rifle. I would stay away from the .308 based cartridges or any other high pressure round.
I think that E.R. Shaw also lists barrels for small ring Mausers. With a decent barrel, bedded and floated in a wood stock, it might shoot very well and is a good candidate for re-barreling practice.
In my piles and shelves of books are a couple that specifically deal with re-barreling Mausers, and so far I haven't found them. Hinnant's book on rebarreling is a good all around guide, but small ring and large ring Mausers are different.
Texas State Rifle Association Life Member
NRA Endowment Life Member
A big fast bullet will beat a little fast bullet every time