Welcome to VMF. You say you'd like to have behind the dash A/C. Are you referring to 67/68 type? Have you seen one on a 65/66 Mustang? I know the kit is available from Classic Aut Air but seems like installing in the 65/66 could be a real bear. The Stangs from 67 and newer were made to accept the in dash (integral A/C) factory option. For example the 65/66 radio goes where the A/C center outlet is on the 67/68. Also that area is crammed with flexible duct/hoses.
I think it would be really neat, not trying to convince you not to do it. Just saying it's likely a tough project for DIY and probably costly if you hire it out.
My '64 1/2 vert. Ordered May '64. D code 4 speed, handling package, caspian blue, accent group, Ford blue manual top.
'68 vert. driver. Owned since Apr '78. C code AT, AC, PS, P disc B, PT lime gold, standard black interior and top. NOS RF fender and left quarter.New top and folding glass.