4L80E vs. manual trans - considering a change

[rcrahn]

With recent talk of gas going to $5 per gallon I'm beginning to consider swapping the 4L80E (mated now to 6.0 LQ9) to a manual transmission in our project, a C20 with 14 bolt 3.73 posi rear.

Some questions I have.....
1. Theoretically speaking, how much better gas mileage will be gained by switching to a 5 or 6 speed manual trans?
2. I do know the very cool 67 Holley shop truck used a Tremec trans. What other trans can be used? Note: I'll keep the LQ9 stock and use the truck for towing occasionally, so I want something stout.

Thanks for any feedback