I need advice for a good street performance/economy slant 6

for a milage motor i would stick with stock valve size buti would do some bowl blending to help improve flow... also install hardened exhaust seats if it is a 72 or earlier head

as for the cam figure a 200-210 duration @ .050, 320-330 lobe w/1.4 rockers, 106LSA

good compression 9.0-9.4

i would look into dutra duals or try to port out the factory exhaust manifold for improved flow also