You can use R134a in an R12 system. No seals need changin'. Just the feed nipple. You will lose %30 cooling and can make up for this with a much larger condenser.
Or buy some Duracool and get ultracooling better than factory. I ran this in my Scorpio and it like the arctic in there. The Duracool molecule is MASSIVE and you will see very,very little loss over the years. It just can't escape like R134a can with its tiny molecule.
http://www.duracool.com/
I'd ship you some of this Martin. It's cheap over here. Hundred pounds worth (including shipping) would do the entire car along with leak stop and new lube. I highly recommend this stuff and it works ACE in Ford systems.