72 duster heater heater box

I hope someone can shed a little light on this subject for me. I have a 72 Duster Im working on. I"ve removed the ac box to install a non ac box. Is there anything I should know? like is a 6 cylinder non ac box the same as an 8 cylinder? nothing seems to match up . The ring on the firewall isnt even close to fitting. What am I missing?
thanks for all the input