I have no experience with your brand of radiator. But when I ordered an aluminum radiator from Summit Racing, the same thing happened. The radiator was narrower than the one in the car and was too narrow for the shroud. I sent it back. Then I bought a new 4-core radiator from a seller on Ebay. It was advertised as a Camaro radiator that would fit a Firebird. Same deal, when I received it I found it to be the same dimensions as the aluminum one from Summit. At this point I caved and cut my shroud. Fortunately, it works extremely well.

It appears that Camaro radiators are not as wide as Firebirds. I don't know where you would go to get the proper part. As I said, I gave up ater strike two.