I have a 73duster,Vintage A/c from firewall forward, original A/c inside. How much Freon should it take? Thanks in advance

2 pounds freon. Or if 134 24 ounces. Could be different depending on model, how old etc. best to check on Vintage website. Freon is quite expensive.

Are you starting with an evacuated system,nor just adding some? Gets a bit more complicated if adding.

Thermometer in vent should get to about 40 degree F if all is well.

For R-134a systems a good rule of thumb is to charge until the high-side pressure reaches 2.2 to 2.5 times the ambient air temp in degrees F. Get an accurate thermometer and clip it directly in front of the condenser. Run the system with the doors open and the blower on high.

So if it's 90F outside, then expect your high-side to be about 200-220 psi.
Usually takes about 2 cans for a completely empty system.

R-134a is now as expensive as R-12 used to be before it was "disappeared". However, a 25 lb tank is relatively cheap (check Costco and Sam's Club) if you plan to do more than one system, and it will keep a long time.

I have read that if changing from R12 to 134a you want to use about 70-80 % of the original amount for best performance. Does this sound correct anyone?

original factory fill was 3.25 pounds. My '71, with a Classic Air system took 3 pounds of 134a.