If you had some cam data for the roller, you might cross-check other modern manufacturers for comparable RPM vs. open spring pressure. Are there part numbers on the cam? Lobe height?
Is the failed pushrod definitely a match for the cup radius in the roller tappet (which may not be stock for that year stovebolt)? I would certainly get the best pushrods, Smith, Manton etc. can supply a 3/8" tapered pushrod that will clear your block and head with a bit of work.
I assume you have stock latest rocker arms (raised fin across the center span)? Some weight can be safely removed from them by hand, not critical but every little bit helps.
