Have you considered staying with R12? On a recent Pantera project I had an AC system that seemed in a state that is similar to your description. After careful thought I pumped the system down and added two cans of R12, and the system worked like new. You can still find R12 cans on craigslist or eBay for about $25 or $30 per can. The system still had sufficient oil to keep the York compressor in a healthy state. R12 likely works more efficiently than R134A in a stock Pantera AC system.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.