i wanna know more bout slant sixes

I don't see how a slant-6 couldn't get mileage that high. However, I personally think every good slant-6 should have a turbo. They are practically made for a turbo (lots of space, intake/exhaust manifolds in same place, etc.) and with the right size, could give you a lot more power without sacrificing mileage (of course only if you kept your foot out of it). New multi-spark ignition, good carb, and free-flowing exhaust would also help. BTW there's an article on Allpar.com about turbo'ing a 225 /6 with a junkyard turbo from a 2.2/2.5 4-banger (although personally I think you could get away with using one a teeny bit bigger).