After market air & heat

Classic makes a nice AC upgrade kit for underhood that hooks to the factory evaporator if you still have it under the dash. Sanden compressor, all new lines and condenser. 134R
I wished I done more research I really didn’t know the blower motor holes was different sizes we just got it off the rotisserie and I welded up the original air conditioner line holes and painted the engine bay so maybe some will send some pictures of their firewall to give me ideas. Would it be worth cutting a non air car firewall and section it into my sons car? Thanks for all your help. KB9GIB