whats the best engine for both performance and mileage??

In response to Rumbles post 30.Stock 99 Magnum 5.9,freebee.Looked good on bores,heads replaced with Cummins stockie replacements.Considered the factory roller 380/360 crate cam.Youtubed it, way big.Punched the numbers,overlap 78(same as a chev 350/327cam). Ended up with a single pattern Crower(284/284 112 l/c,224 @.050).Overlap works to be 58. Idles at 800,sounds like a 340 grind.21inches of vac,at idle.Gave up some h.p.,I will do heads eventually.