I bought a sp2p manifold for the 440/413 industrial head motor I mentioned earlier. Both the intake and heads have tiny ports for the cubic inches, similar to what a 360/ 318 head/sp2p combo would have. 413 industrial heads have 1.88 intake and 1.5 exhaust valve in a big block head!
If your first priority is fuel mileage, then I would start with the sp2p (unless I had something else on hand to try of course).
Just be aware that the combo of the small 302 heads and sp2p intake will limit the rpm of your 360.
I doubt my motor will ever have need to spin over 4k in my application. Peak torque will be somewhere around 2300 to 2400 rpm, and plan to gear the MH to lumber down the road at less than 2000 rpm @65mph.
Whatever else you do make sure you get the piston/gasket/head combo to give you good quench about .035 or so depending upon how much piston rock your short block has. Number one priority in my mind if seeking efficiency and detonation resistance.
