A/C to non A/C change

The big thing is that the firewalls are actually different. You can look up on the net, somebody makes "repop" firewalls and show the photos of how they look. different holes, different shape

Frankly all you really need to do is pull the underhood stuff out and use it as is. If you want you could pull the box out and gut what you don't need, remove the evap, make a cover for the firewall holes for the evap, etc.

The other thing you could do is pull the factory AC out, and buy an aftermarket heater and stick under there.