I took Tech. class in gunsmithing school, from W. Ellis Brown, prior to his writing first edition of his book, and have converted a number of shotguns. I have a 20 ga. mag., sidelock, by one of he Spanish makers, beautifully converted to 9.3x74R by Francotte, so that pretty much shows that even professional big time builders did such conversions, and I have done similar conversions myself, so my opinion is that your 20 ga. is suitable for 9.3 x 74R or anything with similar rearward breech pressure thrust, while leaving adequate comparable chamber steel thickness. Further, the barrels at breech on your 20 gauge, are simply too slim to do a conversion to .405 Winchester; I wouldn't even do it on a 16 gauge frame, but think 12 gauge is more suitable, about perfect, allowing for needed weight between the hands, when in shooting position. As Brown said in his book, shotgun actions are much less robust and lighter than double rifle actions, so that must be given careful consideration always. Your Belgian 16 gauge will easily handle the .45 LC, but is too small for conversion to .450 nitro--much better to go with 12 gauge for that. Remember, the very first conversion that Brown did, was a 16 gauge German Simson action to .450 NE, and he told me that he would never do that again, it is simply too light in weight, requiring the barrels to be too large in diameter, too heavy, forcing too much barrel weight away from the frame, where weight should be. The conversion worked, held up fine, and has shot a good number or rounds now, but barrels are entirely too heavy for overall rifle.