Having torn down and rebuilt Louises 1976 2.8 AC system, duct, dash heater core. Evaporator coil, etc, etc, etc. I will pretty much help as much as I can. First off the mid year pintos and mustang II's have a flange gasket system connect to a metering valve device (forgive me I forget the exact name of it at this time) this is regulated to use R12 only. If you want to convert to 134a then you will need to install a whole new system from vintage auto air or a similar company. They cannot rebuild or adjust te metering valve and the replacement refrigerants do not work well. Freeze 12 being the better of the replacement refrigerants. It is almost as much as R12 these days, so don't bother with it. As for reassembly, the flamnge block has gaskets ( they resemble o rings ) that are not available and I have had more than enough trouble trying to seal this area. I thought I had it andthe gasket blew out after a month. I think I finally figured out the problem. It has held for two weeks, if you havethis set up, email me and I will fill you in on how to fix this. I also have some of the gasket materials I used.
Next if you have the same set up we have it is a very complex and tight fitting duct system and going around the gauges is going to be a frustrating endeavor. There are dampers and flow directing lives that go right up to the vent openings on the center area, so redirecting this set up may nit be easily done without redoing everything. If I started over, I probably would go with a new system from vintage Auto air, one that will work with the cheap r134a. Converting the system to r134a with the existing equipment looses 20% cooling and doesn't work when you need it. So stay with r12 or do a complete tear out and change everything. Dave, send me some pics of what you have and I may be able to help.
Bosspinto@pintostampede.com