I've only used the Vintage Air system and can speak mostly about that - and they are good. There is a size that will fit almost any under the dash or if crowded with other gadgets. They also have one that will fit in the trunk. Without knowing what you are building don't know your size restrictions, but mine was a Vintage Air Gen ll Compact and was installed out of sight in a '30-'31 with a 5.5" recessed firewall along with a fair amount of reinforcing steel - that is until I decided having a/c in a roadster wasn't worth it. While it ended up totally assembled and all hoses run, it was never charged. Below are a couple of photos.

Vintage - they do have a very good reputation, parts if needed are immediately shipped and they have a good customer service. The other companies - several have gone for various reasons, mostly the economy.


Under dash

Under dash

Just before disassemby for paint