Just because the maximum pressure for the 7X57 is now listed as 51,000 PSI or so is absolutely no indication that the #5 Remington rolling block is safe at those pressures.   The reality is that the 7mm rolling blocks were built for the 7mm Spanish which was slightly different than the later 7X57.  Most modern loading manuals say not to use anything but their lowest pressure loads in the 7 mm rolling blocks.  The last batch of rolling blocks were made in 8 mm Label and I have seen a few in 30-40 Krag as well, both of which would suggest about a 40,000 psi maximum.   I've given a lot of thought to building one in 30-30 but, if you have a 7mm with a decent bore, just shoot it as it is  with safe loads.   Probably a better hunting load than the 30-30 anyway.

George