Don't let the low side pressure concern you. The lower it gets, the lower your evaporator temperatures are.

You need to be concerned with high side pressures, cycling of the system, and heat exchange in the condenser. Putting an electric fan on the front of the condenser is a big help. Duct temps are too warm.

The typical, "correct" R134 charge is 85-90% of the R-12 charge.

If you're using an R-12 expansion valve, it operates at different pressures better than R134 will.

The sight glass isn't a consideration with 134a, so don't bother with that.

It is entirely possible that you have an overcharged system. The R12 charge is supposed to be 38oz for that car, and for R134a, no more than 90% of THAT figure.

Overcharging will also bring up the low side (evaporator) temp significantly, while giving high side readings beyond normal. Also be aware that R134a climbs in pressure faster than R12, so your pressure readings are skewed towards the higher pressure refrigerant, which gets to be VERY high pressure in hotter weather.

The overcharging problem could be taken care of first, then see what happens. You want low, sustained low side pressure readings, meaning a low evaporator temp. High low side readings lead to a high evap temp. Buy a R134 low side gauge, and peg that needle in the blue section, preferably towards green. low 20s is good, and possible. After a new expansion valve, that is.

See the VOV results thread that I started. There might be similar technology available for expansion valves. At least make sure you get the right valve for your refrigerant.