Slant Six mpg's?

I would leave your car the way it is. The mileage isn't too bad. It would cost a lot to do the changeover, as well as a lot of work. You can buy a lot of gas for what it would cost you as opposed to leaving it as it is.
My '69 Valiant 4 door with a decent used 225 out of a '75 duster stock everything, 1 bbl, stock 904 auto, 2:76 7 1/4, 195 75 R14 rear tires got about 25 highway @65 mph and about 18 in town.
To compare, my stock '70 Dart 4 door 318, (everything else the same, gear ratio etc.) got 23 highway and about 16 in town.
The 318 car was a lot better to drive overall, especially on hills and passing.
Your choice, but i would rather have the power and 2 less m.p.g.
Just my 2 cents.
Tom.