I couldn't agree more. R12 will vastly outperform R134a in this vehicle. While the manufacture of R12 is banned, the distribution, sale, and use of R12 is legal by licensed technicians, if proper service methods are employed. Try calling automotive shops in your area that specialize in A/C repair. They will most likely have R12 or know who does....I strongly recommend staying with R12, most ESPECIALLY in a van. These vehicles need MAXIMUM a/c efficiency which is provided by the refrigerant for which the system was designed.
It is MUCH less trouble and expense to simply repair any problems the system might have and charge with R12. The price has come down in the last few years due to the rapidly declining demand. The cars that use it are hitting the wrecking yards in numbers causing lower demand, thus lower prices. As a side benefit, you will have a much better performing a/c system.