I'm restoring my late grandfather's 1975 Buick Century (4-door).
It used to have a 350 engine in it but I've replaced it with a newer 455 engine. The problem is the carburetor is no good. I'm looking to get a new 4-barrel carburetor from somewhere (either old stock or new parts if these are still manufactured).

Can someone give me some advice on what to look for and where to find it?