Got the heater box out in about an hour. Core was shot, and there was the remains of a mouse nest in there. Goddamn critters!! It was the damn mouse piss that corroded the core since it was leaking right under the nest covered area.

issed:
Had it re-cored for $35 at a local rad shop. Also found a few of those clips that connect the vac actuator to the air door that's for vent or floor selection at a local dealer. Bought both just in case. Mine was badly busted.
Tomorrow i'll be re-installing everything.
The old core was the original as "Harrison" was stamped into the tank on one side.
It was also about 50% plugged up with what looked like gasket material. Wish i knew how that got in there. The rad shop said that old core had probably 50% heat output, and it was still toasty in the front of that truck.
Oh and Kenny, that core i had must've been for a Non-A/C truck because it was the wrong one for mine yet the rad shop said it's for a chevy truck of that vintage. Since i had no use for it, and the fins were pretty bent up i let the rad shop keep it since i've got no use for it.