Looks like it will work fine, you might just need to shim up the mounting bracket under the tab on the # plate.
Funny thing, I went down to the shop and they didn't have the 450 in stock, when I told the salesman about the project he laughed, and I proceeded to tell him I was a 2 stroke guy all the way, but he still tried talking me into trading my KX in on a new CRf450x, when I told him I'd be glad to ride for free it till it broke, he laughed, until I said "what's the big deal, it'll only be a month old when I bring it back, you should still be able to sell it for 2500 after you replace the motor", then he got a real serious look on his face. I followed that with, "when do you guys expect to get a YZ250 in stock?" "Oh, we have two in the crates in back, I thought you were a 2 stroke guy?" " I am. I was refering to the 2 stroke YZ250." "they don't make those anymore." I should have expected that from a guy wearing a Can Am shirt.