Heater Box non AC

[truckin 79]

What years heater box will fit a 79 without AC? I spent most of the weekend changing bed box quarter and also wanted to put in carpet. My dad had like fake blue turf stuff instead of carpet, well pull that stuff back and see a big rusty wet spot. It looks like the heater core has been leaking and for some time now. Well I look at heater box and it has 2 big holes in it, it looked like duct tape was used at one point to try and seal holes. The things to do to this truck keep piling up. My dad knew what he was doing when he gave the truck to me with all the stuff that needs attention.
PS the spot welds down by the tailgate to remove truck bed fender are a mutha to cut out. I burned up 2 sawzall blades just doing passenger side. I still have to do drivers side after I go to the local Home Depot for more blades