904 or A518 help

forgive me if this has already been addressed- new to the site and went through several pages but didn't see my question answered...
I am building a '74 Scamp with a '86 Police 318, a low shiny driver. I don't plan on drag racing at all but would like to increase my gas mileage with an extra gear. Of course I'm on a budget since I just had to replace my furnace... So has anyone swapped the stock 3 spd for a 4 spd? I am capable of the fabrication -just weighing the cost and overall benefit- oh and the mini tub is not complete and the rear end gear ratio has yet to be determined.

Thanks for your input

Lee