I have needed a heater core for a wile. I was told that my truck was a 64 frame and a 65 cab. This was combined on the title. When trying to find a core....I kept coming up with a smaller one then came out of the truck(6.1/2 x 2.1/2 x 7.1/8. I do have an outside air door that vents to the firwall wiper area. Fresh air intake. My core is 2.5 thick,6.5 tall and 9.25 long. This is what came in a c-20. Im wondering if my cab was off of a c-20 or has anyone else seen this larger core in a c-10? Thanks Grinder4400