Greetings!

If ice is forming on your evaporator core, then obviously the evaporator is getting too cold; the evaporator core should be operating at around 33 degrees F.

The likely cause of the evaporator getting too cold is a stuck expansion valve, although a refrigerant charge level that is a few ounces low could have similiar results.

According to the service manual for the vehicle, this is an R-12 system. In a properly operating R-12 system, you should be seeing low side pressure very close or slightly above 30psi; this is critical to prevent the system from freezing as you describe. high side pressure should be in the 125 to 180 psi range, depending on ambient temperature and thermal load on the system. With a sticky expansion valve, they low side pressure will be dropping too low, while at the same time the high side pressure will be at or above normal: this is due to the expansion valve creating a restriction in the flow resulting in the compressor sucking all of the refrigerant from the low side and pusjing it into the high side. So, if your low pressure is too low and high pressure is normal, then the expansion valve is faulty and will need replacement.

if you have a low side pressure dropping below 30psi, and high side pressure is also low (below 100psi), then the system is low on refrigerant and will need to be charged to restore correct pressure.

I hope this is helpful; if so, an accept would be most appreciated! Thanks!