[-The Stig-]

haha Speed-o-Motive... Where I bought my motor from... Havent gotten a chance to drop in my own 383 so unfortunately I cant say what I get... but...

I'm expecting 10-12mpg through a Th350 Tranny w/ 2000stall and a 4.10 rear.

Now, a 4spd Manual like a M22 Muncie wouldnt help much in the department of MPG... Lack of anything that resembles Overdrive. But like you mentioned a T56 should help cause of that 6th gear.

Although, a 700R4 Tranny might be of use as well. Just use a locking torque converter so you get better highway milege. The 4th gear Overdrive will help you as well. It's also cheaper to swap in a 700R4 into a car that's already an Auto... than trying to convert. Plus, 'cutting' up a old car to convert auto to manual or vice versa just kills the resale value in the world of classic cars. But of course its your car.

If you want maximum efficiency and can afford it. Why not convert to EFI? Edelbrock makes kits, as does Holley. They run about $2000-$2400 for the entire kit. Holley makes a MPFI tunnel ram unit thats just badass, but most likely will not fit under the stock hood of a F-Body. Or most cars for that matter... Its fairly tall, so a cowl hood is needed.

Also, Gear ratio is another thing to consider... If you want a cruiser thats efficient enough for practical use but still has nuts. I'd say don't go over anything steeper than 3.73. With a 3.73 and a T56 you should be able to cruise at 65-70mph at about 1600-1900rpm in 6th gear. It also gives you one hellasious passing gear.

Hmm.. All I can think of at the moment.