Felix,
16 ga. frame is too small, too light in weight for a good well balanced .450 double rifle. The 16 ga. may, indeed, hold up to the stress, but at the same time, it requires barrels to be excessively large in diameter, too heavy, throwing too much weight toward muzzles, not making a well balanced "between the hands" feel to the weight. I am sending you a PM to explain more of the problems in converting 16 ga. to .450 caliber.