After market air & heat

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
So if I'm understanding, you welded up the line holes but left the blower motor hole? If so, theres someone on here that made a plate to fit the aftermarket under dash box to the AC firewall.