New bb build fuel economy

If I were going max fuel efficiency I would be asking the same questions you are. with your target 9:5.1 I would look into a boosted application with fuel injection as well. I believe there are some benefits to this by cooling the charge with methanol injection systems. Yes you pay up front, but with the goal of using the 400 platform.......or......look around for a 5.2 or 5.9 that has a dry manifold already and work that unit. I have not done it personally, but look up some boosted threads on here and see what they are getting for power and efficiency.

my 2 Krona

EDIT: I know this isnt the hard core answers you are seeking ie which pistons, what kind of headwork.....but if fuel efficiency is the goal, big cubes, smallish cam and boosted is one of many ways to get the job done.