Going to a manual transmission of any kind would be a giant leap backwards in technology. If you want better gas mileage (which isn't likely to happen) try fitting a 700R4 overdrive automatic instead of the TH 350. It's almost a bolt-in swap, and there are a few dual-pattern shifters available that will allow you more control over the shift points than simply selecting "Drive" and letting the transmission do the thinking for you. You could even fit a spring-loaded dummy pedal to give your left leg some exercise!

The 350C has a locking clutch in the torque converter that is very easy to make functional with a couple of switches and a little creative wiring. It allows a 10% reduction in cruise RPM at light throttle highway speeds by eliminating the built-in slippage in the torque converter. The minimum clutch engagement speed can also be adjusted by changing a pressure-sensing switch in the valve body that responds to governor pressure as the driveshaft spins.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!