'65 Vert, Can I replace the heater core and how???

Yes, you can do it, but depending on how good/bad the defroster plenum is, you might have to loosen the AC from under the dash, and pull the right side around some. I'd cut the old heater hoses off at the firewall. After you take the nuts off the heater studs under the hood, take out the sheet metal screw that faces straight up behind the glove box, that holds the 90 degree bracket to the cowl. You should be able to pull the heater back and down enough to be able to get to the cables. After the heater box is on your bench, take off the clips by prying them apart and over the ridges in the box halves. If you just pry them straight up, you take the chance of taking that little ridge off and the clip won't have anything to hold onto when you're putting the heater back together. Once you have it apart, you should replace all the foam pieces, new heater core, and new hoses too. If the defroster plenum is old, get your self a new reproduction one, they're ok pieces. Slip the new heater hose through the firewall and hook up the hoses right before you hook up your cables and push the blower motor back in where it goes. Make real sure you don't kink either hose, or the hot water won't circulate. Good luck, hopefully this might help.