Torque for that application is King, as well as keeping highway RPM in the meat of the torque curve, usually around 2600-2800 RPM.
MPG benefits when using as little throttle opening as possible while pulling.
While my 408 made very good TQ in a Duster, it didn't compare to an RB.
Thermoquads are great in that application, 2nd best a vac-secondary.
I'd use the existing MH converter. Shift kits are a good thing, as they reduce the shift overlap; doesn't need anything racy.

Since the OP already has the MH 440/727 and the 318 is tired, the path forward seems pretty obvious.
Make sure you can cool it, and add an ATF cooler. I like deep pans on everything.