The 390 cfm Holley would probably be a closer match on a 250, but on the other hand,a new Holley 390 cfm carb is going to cost a LOT more than the $25 bucks you paid for the 7455. So if you wind up a wee bit over carbureted, no big deal. 450 cfm is still pretty close to what a 250 would need, depending on what sort of modification has been done to the 250.
