68 GMC gas mileage - 305 or 350?

[thaubol]

Hey -
This is my first build and pI'm struggling with the decision on what to put into my truck to achieve the best fuel economy.
It currently has what I expect is its original transmission with a 350 and some heavy rear end gears - I'm guessin 4.11, with 215/75-15 tires on the front and 275/75-15 tires on the back. I recently bought a t5 out of a camaro that I plan on using in the truck (in the interest of fuel economy). I also have a 88 monte carlo ss with a 305.
One dilemma I'm facing is which motor to use. Obviously, the 350 is able to have more power, and being in an older, heavier truck, that might be the better bet. But in order to attain the best gas mileage without running the motor lean I was thinking that the 305 might be the way to go, as long as I can find a way to up the torque so the truck isn't struggling while accelerating.
Another question I have is what gears I should use in the rear end. Most everyone is saying that a 3.08 is going to make the trans hunt for od on the highway, so I was thinking of going with a 3.43 or a 3.72? I'm pretty sure I'll be using 255/75-15 tires.
This is my first post on this forum, about my first build, so maybe i should shed some light on myself and my truck. I'm currently 21, I bought my 1968 GMC C15 last summer and have fallen in love with it. Any suggestions on any part of this build would be much appreciated as, like i said, this is my first build and am relatively clueless.
Thanks, TC