Putting a factory unit in is going to be a TON of work.
Besides sourcing the parts, you'll need to make changes to the dash and firewall... You ready to pull the dash? Honestly, if I were doing that conversion, I'd look for a parts car... Just sourcing the dash pieces that were missing on my 300 was a PITA. I've got way too much time and $$ in mine... and that was a factory air car that had the A/C removed years ago.
You would be lots of time, frustration, and probably money ahead to get an aftermarket unit. I have no experience with Classic Air so I can't say one way or the other about them. I have read that Vintage Air is really good, again, no personal experience with them.