[Angel And The Wolf]

you might consider the Prius simulated Atkinson cycle engine. The Prius engine is an 1800 CC 13:1 four cylinder, but it's only 13:1 on the power, exhaust, and intake strokes. On the compression stroke, it pushes air back out the delayed closing intake valve, so that the effective compression stroke is 10:1. this allows the engine to act like a 1384 cc engine, (346 cc per cylinder) with an extra 103.75 cc of expansion stroke per cylinder, salvaging the heat energy that would normally waste itself out the exhaust manifold.

Raise your compression to 13:1, and install intake cams that will hold your intake valves open 41.54 degrees past BDC. Use 87 Octane regular.
You need port fuel injection, so you can delaying injecting the fuel until after the intake valve closes.

Using your 350 as an example, this would effectively give you a 269 Cu In V8 with the mechanical efficiency on par with the Prius engine.