I think 65 was the was the first year for factory AC on the A Bodies so that might have something to do with the difference (if there really is one) in the heater cores.
My '64 has A/C but I guess it is factory installed, then?
I haven't taken mine apart yet, partially due to having the A/C unit below the dash. Since I want to replace the windshield, possibly remove the entire dash, swap to a 4-speed and fix the heater core, I figured I'd take it all apart at the same time. 8)
