It could be that the evaporator is fighting the heater core. That'll make the low side abnormally high. Shut the water off with some pliers or vice grips.

I thought the 91 had a mechanical fan clutch, not that it seems to be the problem, unless your high side is really high. Which might be the case with your gauges being off and all.

I can't comment on what the site glass means, never use it. I know the general theory, but you have a different refrigerant in it now.

Yes, it has a mech fan clutch, but an aux fan in front of the evaporator kicks in as soon as the AC is turned on.

I'll drive it tomorrow and see what the results are with letting some out. Then I'll try your idea of clamping off the heater core hoses.