R134 has similar properties, rule of thumb is around 100 Psi over the ambient temp plus or minus. Low pressure can be around 20-40 psi depending on humidity and if its an orifice tube system or expansion valve system. My numbers may be a little off because most of my experience is with newer cars, but the principle is the same. The high side pressure should rise slightly with higher rpms and the low side should suck down slightly, all depending on air flow over the condenser of course. Charge on the low side with the engine running until the bubbles disappear in the sight glass. It might take a little bit for it to suck it in, but it will. When bubbles disappear, close the manifold gauges valves and re check pressures. I never liked retrofitting to r134, if the system was designed for R12, I would always try to use R12. R134 has smaller molecules and has a tendency to leak where R12 might not have leaked. Also, if your hoses are original, it may leak thru the rubber. R134 is supposed to have hoses made of a different compound referred to as “barrier” hoses. R134 is being phased out now in lieu of the new 1234yf, there is no retrofit for this new refrigerant.