The difference between 31,5gm 70mm and 39gm 76mm Rottweil offerings is approx. 250bar. Oh yes, you will have to lump on extra pressure generated when 76mm cartrige is fired from 70mm chamber, but I don't know what that value is.
PS. H&H just reincarnated that thingy lobbing 48gm bullet at 320m/s and 8lb 76mm version will be available soon, but that seems woefully inadequate for Mr. Buffed.