Most of the A/C repair places just want to let their machine run through its automated cycle, and they often have no idea what should be showing on the gauges. The automated machine evacuates all the R134a, then pulls vacuum for a set time (usually 30 minutes), waits a preset time to see if the vacuum holds, and then charges the system with the selected amount of R134a. If everything goes well, the "technician" only needs to know how to connect the hoses and look up the R134a amount in a book so he can set the machine.
In your case the book they have won't list an amount for R-134a since the truck originally had R-12, so that can bring an inexperienced "technician" to a halt. However, all he has to do is look up the R-12 amount, and then multiply it by 75% to get the approximate R-134a amount. For example, my '75 with A6 compressor used 3 lbs, 12 oz of R-12 (60 oz total), so the expected R-134a charge will be about 45 oz (2 lbs, 9 oz).
For a conversion it is usually best to start out with a little less than the recommended amount of R-134a and then watch the gauges while you add 1 oz at a time. However, that means the technician also has to understand how to interpret the gauge readings, which will vary based on ambient temperature and refrigerant charge.
Bruce
