BBF, I have really been biting my lip and staying out of this one. I think the 445 is a great little cartridge and would make a neat rifle, but I wouldn't rechamber a perfect 44 barrel for it, especially if I had a 450.
The real beauty of reloading is being able to take a case like the 450 and get 44 Mag to start level 458 Mag performance. Any 458 cast bullet of 300-350 grains pushed by either light loads of pistol powders or full case loads of very slow powders like 4350 on down through 870 will be perfect duplicates of cartridges like the 454 Casuel, 480 Ruger and 475 Linebaugh. For years I approached reloading from the standpoint of getting maximum velocity from a cartridge. Today maximum performance means something different to me, it is exploring every level of performance a case has to offer. Trust me, 44 caliber 200 grain bullets at a mere 800 fps are an absolute pleasure to shoot. I can shoot them all day, did so many times this past summer, and when deer season came was as in tune with my rifle as it is possible to be, which made it possible for me to be able to take some quick shots that would otherwise have been "there it is...there it goes"
They will be making that reamer for quite a while yet. It may well be that the 445 SM is just what you are looking for, but I don't think you are giving your 450 a fair shake. Try it with some light loads. Use Springfield Trapdoor data, you will be quite fine with them. If it can be done in a 45-70 case, it can be done in the 450 as well.
I speak on this only because I would hate for you to tie up your money in the 445, when you already have one. The 445 won't do any thing your 450 won't do, and your 450 will do plenty your 445 will never do. One reloader to another, give your 450 a test run on its full span of capability. Everything you learn from the 450 will give you an edge taking the 445 to it's ful range of potential.
Good Shooting, JP