I have a '95 Mustang that has been sitting for over a year with a three quarters full tank of gas. I just pulled the old engine and am in the process of installing the new one. Should I drain the old gas or would it be alright to treat it with Pri-G and use it?

 

One year old gas isn't the end of the world,, it should still be OK so long as you just drive conservatively with it until you refill the tank with new gas.

..the bigger concern might be thatthe tank was only 3/4 full for a year.. you might have picked up some water condensation in that empty space that woulda screwed up the gas even more. Personally I'd drain it out.

 

Drain it. Being that old, it has probably turned into varnish by now. If you run tight clearances with new engine, you may stick a valve.