You're both correct. The Victoria was introduced in '51, but I prefer the simpler trim of the '49 and '50, so I made a phantom '50 Vic out of mine. Here's the rest:
Depends how you go about it. Yes a 51 bumper will bolt on as long as you use 51 brackets however, the contour is different than 49/50. I used a 51 lower valance being mine is a 51 and put 50 bumpers on it. That required re working the front edge to match the bumper.
While I don't recall which brackets I used, I do recall that the '51 gravel pan didn't like the stainess piece that is visually the bottom part of the '50 grille. Luckily I found a NOS '50 pan, and everything went together nicely. The OP wants to go the other way, though, and put a '51 bumper on a '50. I don't know which parts may require tweaking, but I have no doubt that the installation is possible.
It was always common to 'update' your car with later parts. My Crestliner has 51 bumpers and headlights
I guess I'm more of a reactionary. I have a '51 pretending to be a '50, and a '56 disguised as a '55. The car in my avatar is closer to the "updated" idea. It's a '40 which will be wearing a postwar style soft top.
Thanks to everyone for the replys, I found a good set for a 51 with no brackets but I think I`ll stick with the 50`s if I can find a set at a decent price. I still have a lot to do to the old girl..the next thing is pulling the engine so I can repair the crossmember that was cut to put in the 351. I like the late model engine & transmission but I like keep the rest of it close to stock.