Propane makes an excellent refrigerant
the issue with highly flammable refrigerant is that it is under pressure... sometimes 200 to 300+PSI in running condition. if you leak, and you have a sparkplug wire wire that is grounding out/ shorting that could make your life more interesting.
The reason that *I* would not use that stuff is because no shop will touch your a/c system once you convert over to any kind of blend, or if they do it will cost you...those blends will contaminate their recovery and R12/ R134a supply.
Granted, you might be just going for getting it nice and cold on a budget and this truck will probably never see a shop (at least for a/c) again in its life, but it still limits you in what you can do if this stuff doesnt work out, and if you do want to convert to R134a you will have the R12 and R12a blend that you have to purge from the system..
for a truck as old as yours, i would go ahead and replace the hoses with new barrier type hoses that will work with 134a, replace the condenser (or flush the living hell out of it... time consuming and a pain, I have been there when I rebuilt my a/c) and replace *all* of your o-rings. take the compressor out and get as much of the oil out of it as you can and put some r134 AND r12 compatible oil in it. be sure to flush the evaporator coil as well.
then replace the accumulator/ dryer and orifice tube and put it all back together again and put the R134a retrofit valves on.
pull a vacuum on it for an hour or so and refill with R134a. if you do it right, you will probably have cold a/c for the rest of your trucks life
check this write up on resurrecting your a/c:
http://therangerstation.com/forums/showthread.php?t=144542
This is for a straight R134a system, no retrofitting, but the principles hold for your truck as well. just remember you will need to change the oil out to a type of oil that is compatible with both R12 and R134a. PAG oil will sludge your system up.. Do some research and you should be good to go.
AJ