If you think the OD will get you much more MPG's, you're wishful thinking. It will get you more speed, but unless you drive it constantly, it won't pay for itself in fuel savings. Go with the durable TH350 or TH400 for a working truck.
Just my opinion,
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!