Actually, the radiator support is the same between the GS and Skylark. The difference is the early '65 Skylarks (before Jan '65 when the GS was released) had a downflow radiator in them, which has a different core support. Starting in Jan '65, all the Skylarks switched over to the crossflow radiator design that came out on the GS and used the same core support as the GS. That is a loose date--I have heard of cars built in Dec '64 with the crossflow and I have seen some V6 cars built in Jan that still had the downflow (probably were still just using up the old inventory before switching over to the new design).
The core supports differ in both the bottom and top crossmembers. Bottom is shaped differently and the top has different holes stamped in it. The dowflow design actually has weld nuts made into the top crossmember, whereas the crossflow design uses extruded holes for the bolts.
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