Because the compressor is locked up, and because it's more expensive to repair an R-12 system than to convert it, I'm going to swap to R-134a while replacing the compressor.
Note that there's plenty of room to store things under the hood, like jumper cables, spare water/coolant, ATF, brake fluid, etc.
A QUICKER & usually cheaper way to do this conversion is to find a '94-96 factory R134a system still sealed on a junk truck with the same engine as yours, and lift it out as a unit, including the underhood HVAC box, and the hood latch support bracket; and swap everything without opening either refrigerant system.