I had the AC in my '88 Mustang converted to R134a. I also had a "smart VOV" installed at the same time:

http://www.sherco-auto.com/smartvov.htm

I have no idea how much the smart VOV helped, but it definently cools adequately....

By the way, Ford says that replacing the accumulator is not required when converting from R12 to R134a..

Brian