I'd use Classic Auto Air. They have the aftermarket HVAC boxes, rotary compressors, brackets, etc. I used their Sanden compressor kit and had them make custom lines for my 1973 Charger and they were extremely helpful.
I talked to the support guy, Ron and he knew his stuff. I'd call him when I was at my local A/C shop getting the fittings installed with a question or needing a part and he'd always answer (phone or email) and have whatever I needed shipped out the next day.
If you car was non-A/C originally, then yes the firewall is different so you would have to drill new holes and weld up others as already stated. Since you are looking at some mods to the car anyway, I vote getting the aftermarket box and going with a rotary compressor. BPE sells them but Classic Auto Air does as well and they are cheaper and will have
everything you need. I think they even have some 'perfect fit' systems for your car which supposedly drop right in (at least to an original A/C car). You will probably have to add vents to your dash if it was a non-A/C car.
Aftermarket systems will also most likely use electric servos to open/close the air doors inside which is worlds better than the original vacuum actuators. If you have an engine with lower vacuum or hit he brakes really hard, you will have the doors returning to their resting positions momentarily with the old actuators.
The newer electric servos don't have this issue.
