NO!!! This does not seam right. In fact it seams to be total BS! My initial reaction is to tell you to get your radiator and whatever money you can back from the shop. Something just doesn't seam right. They should have seen the cracking before they replaced the core (if there really is cracking) and maybe a complaint to whomever regulates repair shops in your state may be in order.
I would want them to show me the stress cracks and show me that this is where it is leaking. Typically, in the early '70's radiators were 18", 22", 26" & 28" wide with the 28" typically being in Imperials and the 26" typically being in A/C cars and cars with heavy duty cooling. A factory 440 car with A/C should have a 26" radiator and I believe that the '70 - '73 26" radiator tanks for big block cars were very similar although the part numbers may have changed over the years but they would have to get aftermarket tanks that should be the same for multiple years. Before you do anything further I would definitely get the part number off the top tank to make sure it is the correct radiator to begin with.