SuPer SiX Slant Signet!

If you want mileage, go with the 170. You should be able to get over 20 MPG with it.

I would build the 170. Get the head ported (I read an article that stated the slant 6's respond very well to opening up the bowl part of the ports). Then, you could try a 2 or 4 bbl manifold and carb set up with a good free flowing exhaust. If you don't like the 2 or 4 bbl, you could always go back to the original 1 bbl. For economy, maybe try to find a cam with about 240 - 260 duration and like .400 to .425 lift. My opinion if you go over .450 lift, you will lose some economy.

Larry Shephard used to recommend the 252 cam for the small block for best economy. I believe that it had like .400/.425 lift. This was the stock 360 2 bbl cam. My dad used to drive a 76 Charger with a stock 360 and got 20 MPG highway.