I once saw a pre-64 Model 70 which had been converted to .416 Rigby by Griffin & Howe. I asked Paul Chapman, head of gunsmithing at G&H, about that conversion and he said that they didn't recommend it now.

Actually, with the number of true magnum length Mauser type actions and rifles floating around, in particular the CZ 550's and BRNO 602's, at quite reasonable prices, there is no reason for such conversions.