heater core leak?

In pre 73 models the firewall is different. To put in factory ac/ you have to cut away metal and add some holes. To remove factory a/c and install a heat only box you have to add filler metal back and plug some holes. I dont know if this firewall issue applies to 73 and later or not. I do recall several years ago a member had a tough time finding the correct heater core for his 74 model with a/c. Something about the tubes was different from other models. Good luck with yours.