Trying to bring the A/C back to life.

I bought my 70 dart last summer and was so excited to own it I didn't mind not having A/C. But I would really like to get it going this spring. The slant 6 runs great and has had a full tune up performed. I've logged about 4500 miles on it since owning it. The drivetrain seems solid after it sat for 18 years. Last fall I replaced the heater valve, heater core and all of the HVAC box seals along with the vaccuum lines and push button switch. The heater is HOT and all of the doors function as they should. So hopefully I won't have any issues with the evaporator.

My car has factory A/C on it and it's all intact and hooked up. But it didn't have any Freon or pressure in it. I just bought the 134a conversion fittings, 134a compatible dryer, New O-ring kit, and expansion valve. I jumped the compressor lead and the clutch kicked on and the compressor spins. So I'm going to chance it and hope it's still good. Changing everything out was is straight forward and simple.

My question is how/should I flush the old oil and possible trash from the system? If there is anything.
How much oil should I add since I'm not replacing any major items? I bought some Esther oil so it's backwards compatible to the R-12 stuff.
Would my 1970 dart have an EPR valve? I've found a bunch of conflicting info about the EPR valve. Some say keep it, some say toss it. If it is really beneficial to toss it, I will and install a thermostat. But if its fine then I'll just let it be.

I've done A/C repair work in the past. But I'm not a pro so I would like to get my info from a pro before I waste money on Freon.