Wow... I bet gas mileage could easily double with efi. My 74 with a 3.6 gets about 25 - 27 mpg on average and I tend to drive in a very spirited nature. A side note is that my car only weighs 2300 pounds to so that could be a contributing factor to the good gas mileage.
I am going to be switching out the 3.6 for an efi 3.3 or 3.4 turbo I am hoping to get at least somewhere in the low 20's for fuel economy.