^^^^ X2 on this, it's a major project. The holes have to be sealed that went to the AC lines, the heater hoses are in different places, and IIRC the heater fan is also in a different place. I considered going down this road till I get a good look at what was involved to convert. I kept looking for an AC box so I could swap it out directly.
The entire fan motor goes through the firewall on an A/C car and has a few extra holes for the lines and drain, on a non-A/C car only the motor itself goes through the firewall and there is a plate that has 4 nuts that hold the heater box on. So holes need filled, and an entire section of firewall has to be removed and a new section welded in...
It can be done. Heater core holes are in the same spot. As stated above extra holes need to be filled in and the blower motor hole needs to be smaller...