Maybe not 134a, but definitely should take FR-12 (
Harold Electric Co. specializing in Ag & Mobile Products)
Biggest problem with the switchover is that the Thermal Expansion valve will have to be replaced to make best use of 134. A pita, to say the least. If you do the switch, I recommend looking at replacing the entire evaporator with a new Griffiths unit. A thing of beauty and substantially larger. Besides, if you pull the a/c box you might as well properly renovate it.
I'd also take a good look at that rusty pipe coming out of the compressor. One rust-through pinhole and the refrigerant is gone.
As for usefulness, I added air to my '71. Either that or it would never get driven in the summer. Removing a few pounds from the tail can't be a bad thing, but...