Waidmannsheil,
I would guess that your 12 gauge Merkel would stand up to about any reasonable 450 load. I would be more concerned how the firing pins fit in their holes. If there is not much room around them I would not worry about it at all.
My reasoning for going with the 45-120 loading instead of the 450 3 1/3 inch nitro load is the same reason you stated. I can buy Norma 45-120 brass much cheaper,($40.98 for a box of 20), or use cheap Norma 45 basic brass,($24.95 for a box of 20),to get my loads done. A set of loading dies in 45-120 3 1/4 inch was $51.61 from Huntingtons. I bought 140 rounds of new Norma brass for an average cost of $1.54 each. How much will your .450 3 1/4 inch nitro cases cost you?
I'm sure I can approach .450 nitro speeds in the 45-120 without going over pressure with these cases. If I don't quite get that much speed, I will still have a flat shooting load that will do anything I need to do with it, at less than half the cost. I do not need, nore will I be loading any 500 grain bullets, although the caliber will handle them. Most of mine will be in the 350 to 400 grain weights. Case capacity is, for all intents and purposes, identical to the .450 3 1/4 inch nitro, so there is no need to worry about running out of case capacity.
One of your biggest problems in the .450 will be the same as mine, what powder to use and how much of what kind of filler? I intend to use backer rod in mine for a filler. I have already used it successfully in .577 cases and .600 cases. If I was not using the backer rod I would consider dacron as a next best choice. Good luck with your conversion project. Bob