Sorri about that, i got the 13inch model for 39 bucks and free shipping
i then asked about radiator cap, matching unit ,and he said its on sale 15 bucks and free shipping so i got it too.
looks to me like either will fit fine.
The guy thought either one would be fine as far as system capacity for a 65 mustang.
They are out of 15inchers till Monday's new shipment comes in. I ordered from Rodworx