Heater core fins is a possibility... wish I was easy to check.
No help on your problems but how much of a bitch is it to do the heater core? I need to get after that on mine. Not only does it leak but none of my controls work rigjt
With your heat issue is the rubber band like connector between your heater box and dash vents gone? In my 78 it didn’t have one and there was a large gap (like 3/4”) between them and it was just Blowing all the air out under the dash