Dan, I put a 4 row in my '66 Special. I think the rad was spec'd for a '67 GS 400, it was around 1998.
The 65-6 GS used wider tanks that would accept 3 or 4 row cores. Most of the small blocks used narrower tanks except for some wagon and HD cooling versions which used the wider tanks like the GS.
The wide tank radiators used a different top plate, the insulator cushions were different too. They mounted directly in the lower saddle of the rad support, without brackets. Insulators are avail from CARS or Steele.
I have an aftermarket aluminum Griffin radiator in my GS, 27.5" x 19" size. There are versions with in/outlets that fit Nailheads and another version that fits the 300-340 and 400-455.