318 heads

An example of mileage type of camshafts;

High Efficiency Hydraulic Flat Tappet Cam - Chrysler 273-360 245/255
Or
High Energy 206/206 Hydraulic Flat Cam for Chrysler 273-360

But I really suggest calling up a few cam grinders and asking for there help on what would most probably be a better cam for mileage than the ones I listed.
I like the Voodoo high efficiency stuff. The problem would be that they would run so much better than stock, you'd have your foot in it all the time. There goes the mileage. But that'd probably be true of the stone stock motor with thermoquad and headers. Make no mistake, that would be a tire smokin sumbitch.