The .243 Win and the .260 Remington run at a bit higher pressures than I'd put in a Spanish Mauser (93 and 94 actions). You can look at the selection of barrels that Midway supports for the small rings to see what they think are appropriate.

If I had a Spanish Mauser action, I'd put a 6.5x55mm barrel on it myself. Or stick to the 7x57mm which is actually a great cartridge with a lot of bullet choices for you.