[comptiger5000]

With the higher compression, you might have to adjust cam timing a bit, as well as ignition timing, and possibly run slightly colder spark plugs. It'll probably be fine, however, and can definitely be made to work, although 89 might be a little closer to optimal.

If you want to keep your TH350, being that it lacks torque converter lockup, build the engine to have the power band down low, and run the lowest stall torque converter you can.

Even with the non-lockup 3 speed auto, I could see your MPG goals happening. Of course, a 5 speed or a newer 4 speed auto w/ lockup (beefed up 700r4 maybe?) would help a good bit. With your current tranny, I'd consider dropping to 3.23 gears to get the highway RPM down a bit, although the 3.42s aren't terrible. Depending on how extreme you get with the mods, I could see anywhere from 17 - 24 mpg on the highway being possible.

Regarding the current carb, being that it's square bore, it's not optimal for MPG. A switch to a Quadra-Jet or other spread bore 4 barrel with good flexibility for tuning will give better mpg under light acceleration and when cruising at light throttle. A good, well tuned spread bore setup can come close to EFI for fuel efficiency.

Not waking the neighborhood is a function more of the muffler you choose than anything else.

Your goals sound similar to what I'm planning with my Jeep, except that I am willing to use 93, as it already needs 91 minimum, and it's EFI. Of course, my Jeep is as heavy or heavier than your car, and far less aerodynamic. Plus, the 360 isn't known for mpg in any form, but I still get about 12 - 12.5 mpg around town and 19 - 21 driving slow on the highway stock, so I figure I can probably improve on that a little. Of course, once it's retired from DD duty in a few years and gets a (possibly twin turbo) 408 stroker, all bets are off for mpg.