The early 396/325hp engines with stock compression and camshaft are not pump gas friendly. That being said, how you build it and what gear ratio you run really depends upon how much driving you want to do and what your tolerance is for using high octane fuel, etc. A 3.55 rear is not a true highway gear, but it is not that bad as an all around compromise. I would also consider what tranny you have...if it is an M20 then highway gears like 3.31 or 3.08 will be ok. If you have an M21, those lower gears will be a little "doggy" off the line. One of our L78 cars has an M21 with 3.55's and first gear lugs a little. Keep in mind rear tire size will also impact the final gear ratio...you could go with a 15" tire easily on that car and keep the 3.55's...part of the fun of driving a big block Chevy is hearing the motor wind up a little [img]<<GRAEMLIN_URL>>/
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I would be inclined to consider some "non-visible" engine upgrades that improve the cylinder heads along with a modern hydraulic camshaft. If you are planning to drive a few hundred miles a year...then don't worry about the gas...buy the good stuff and enjoy the car as it was meant to be. If you want a pump gas/highway friendly car...take your daily driver out for a ride...if you want to have some fun...nothing else sounds or feels like a big block when you hit the go pedal.
wilma