ducmarc,
It's not a question of how big a shell you can stuff into a 16 ga. frame, it has more to do with the weight of the finished gun and how much recoil you want to contend with. Do you want a light handy woods gun in 45-70 or a truly hard hitter like a .450 N.E. or a 450-400? Much also depends on the general overall condition of the gun, just because the barrels have some rust does not mean that it is unfit to convert, but it also points out that the gun may have not had the best care over the years and there may be other problems inside.
I feel that most 16 gauges with three lockups are good for about any .45 caliber but would not want to build a .450 3 1/4 inch Nitro Express on one. Ellis Brown did that once and said he would never do it again, not because the gun wasn't strong enough, but because being built on a 16 gauge made it too light. There are many hidden factors involved with building a conversion gun that most people don't know about or even think over. Because these guns are built with gently tapered barrels, the barrel size at the end of the monoblock predetermines how big the barrels can be at that point and roughly how much the barrels are going to weigh.
If you want a good shooting gun, you have to balance not only the weight in the hands but also the weight of the gun against the amount of free recoil. If either of these balances is lacking, then your gun is not as pleasurable to shoot. You can't go from a gently tapered monoblock right into straight tapered barrels, if you do that it will look terrible. They must be properly contoured and that determines the overall weight of the gun and how the barrels balance with the butt stock and action.
One thing you can do is to check out barrel manufacturers barrel profiles, some will state the weight of the barrels. If they don't you can always email or call them and they will be glad to tell you what each one weighs. Take that times two for two barrels and add in roughly one pound for ribs and sights. Take that amount times two and that will be about what your finished gun will weigh, assuming that you have balanced the barrels and forend against the action and butt stock. I hope this helps you figure it out, there is much to consider before you get to the final answer. Good luck!
Bob