i have searched high and low for a new core for my 77 620... i finally decided to pull the whole unit out, 3 bolts and the couple hoses (it took me 5 minutes literally to take it out). And i put in the following Danhard universal heater...
http://www.jcwhitney.com/Under...7;0;0
It was a super easy install, and the heater works better then the stock heater i had in the truck (when it did work). It hooked right up to my defrost vents and all.
You can buy the switch kit seperate from the heater if you like to control your heat. i just hooked mine up to a toggle switch since i only need the heater 2 months out of the year ( i live in the mohave desert)
If you decide to do the Danhard Universal heater like i did, shop around a bit. i got mine for $150. JCWhitney is a bit steep on their price.
Good Luck!