I just bought this heater off ebay, I think it is a firestone brand from what I have seen in one post? Were these factory or dealer installed? The heater core outlets and mounting bolts line up perfectly wtih my firewall so I would guess this what my truck had. I cant afford an A/C unit right now but have to have at least a heater and defogger so I got this one to use. Look correct?
Back when, heaters were installed by whomever needed a heater, not necessarily the dealer. Lots of aftermarket brands. Since your truck appears modified, the heater being correct should not be an issue. A working heater in your area would be a requirement.
As late as the 1960s, heaters were optional. I know of a 1960 Chevy Impala, top of the line, that had no heater.