Ok, just to clarify:
"All you need to do is suck out the R-12 and oil, replace the oil, and fittings, and add R-134a".
NOTHING is further from the truth.
These trucks do not convert all that well, from personal experience.
To *properly* convert it (for ideal performance), you need to replace the fittings, suck out the R-12 and drain all the oil from the compressor. All the old oil needs to be removed, because it is not compatible with the PAG oil in the R-134a systems. When they mix, they become acidic, and will slowly destroy the A/C system.
You need to replace all hoses. R-134a vehicles have "barrier hoses" which prevent the R-134a from escaping, as the R-134a molecules are much smaller than the R-12 ones.
You need to replace the orifice tube with one designed for R-134a. R-134a runs at higher pressures than R-12, and R-134a parts are required for optimum performance.
You need to replace the Accumulator/Drier (silver tube by the firewall), as whenever the system is opened, it should be changed.
You should replace all the O-rings as well, and verify the A/C compressor isn't leaking from around the compressor seals, clutch, or case.
Once all this is done, oil the compressor, and put the system under a vacuum for at least 30 minutes. If the pressure doesn't drop, add R-134a. You only need 80% of the R-12 charge.
From experience:
R-134a simply does not cool as well as R-12. With R-134a, expect 40-45 degree vent temps, providing the system is working as efficiently as possible. If it isn't, expect 50-55 degree air. With R-12, we're talking vent temps in the upper 20's, lower 30's. It also takes it a lot less time to drop from 90* to 30*. It is more efficient than R-134a, thus requiring a smaller condenser. When R-134a is added, the system has to work harder due to the higher pressures, and has less than optimal airflow from a condenser that is undersized, preventing the temps from getting as cold as they could be, and as quickly.
Your best bet, since the truck is simply low, is to buy a few cans off eBay. Look for sellers that will accept a letter saying (basically "I, William D. do hereby certify that I am purchasing this freon to re-sell to a certified technician for installation."
In all honesty, the sellers don't give a sh*t who buys the R-12, as long as they can make a buck. eBay forces them to have the "must be EPA certified or resell to certified tech" in their listing, to eliminate any liability.
I should add, for those still considering switching to R-134a, I'm currently running R-12, and even with literally 50% of my condenser fins bent shut (working on getting a comb to fix that), I'm getting 40-44* air. Sometimes it's even in the upper 30's. I'd imagine once I double the airflow through the condenser, I'll get another 8-10* air drop. My A/C is so cold I have to turn it off sometimes on 90* days just to thaw myself out a bit.