Hi There!! Newbie with questions.

For a change in engines, it's ethier a correct K-frame swap or a swap kit from http://www.engine-swaps.com/ for less money. Excellent instructions are included.
Your goal of high torque is best and easiest to achieve with a 360. The extra throw of the crank is great in torque production.
I had a '79 Dodge Magnum (Think Cordoba with a different nose and hood if you don't know what it looks like) with a 360/904/2.76 gears getting 20 mpg's.
The only mods were a 2bbl. swap to a Carter 625 on a factory intake, MSD, Dual exhaust of the manifolds and a viscous fan.

Theres only 1 4spd. manual gear box for the Chrysler line up. It is the A-833. Theres one varient, the overdrive version. I.D.ed by a case mounted lever fliped upside down. O.D. versions came in iron and aluminum. Theres 2 front bearing retainer sizes. I think you would want to skip the weaker O.D. unit to handle high HP & torque.

I believe you can achieve your goal of 20+ mpg's.