I have a 1967 F100 with a 300 *** 6 cylinder and a four speed transmission.  I would like to swap it out for something with better gas regulation / mileage and possibly convert to an automatic.  Does anyone have experience with something like this?  

You willing to go diesel, or you want to keep it gas? I know a lot of guys like the Cummins 4BT in older trucks. Gets great mileage, and you can tune it for some decent power.