People have said on here that some new radiators for Cubs have too short a filler neck, and the radiator cap rubs on the hood.
They have also said that some of the bolt holes didn't line up at the radiator base.
They have said that the Clancy brand fits well. I have not bought one, I can't say from seeing it myself.
TM Tractor sells the Clancy radiator, below is a listing, you can look at it.

http://www.tmtractor.com/new/co/671fp.htm