Non-A/C Heater Box

My 73 came with AC (first thing I ripped out!) and I picked up a nonAC heater that came with the chunk of the firewall that the blower motor sticks out of and the guy told me that I would have to cut out and weld in the firewall chunk in order for it to fit.
Your nonAC heater sounds just like the nonAC heater I just rebuilt and put in my girlfriends 72 scamp.