I had responded assuming that Isaac wanted to do a project on this particular truck. Why not? He has little in it. At that price it is probably on a salvage title and his resale value is junk. Perfect project vehicle. Even if you foul it up, you aren’t out much.
A 300 Six in a three-quarter ton truck tells me he probably has at least 4.10:1 gears and probably 4.56:1 gears. He’s probably wound out at 60 MPH.
Both Ford 8.8 and Sterling 10.25” axles can be fitted with 3.08 or 3.31 ring and pinion sets. I use a 3.08 and find it to be quite satisfactory. Correction to earlier post: search
Randy's Ring & Pinion. The Differential Experts. Auto Parts & Services. for parts.
His OEM tranny probably does not have much of an overdrive. Maybe 0.8:1 if any. The T-56 found in Mustangs has a 0.5:1 overdrive. The low gear is low enough (2.97:1) unless you are starting heavy trailers on steep hills. The torque rating for a T-56 is enough that either a 300 Six or a Cummins 4BT do not produce enough torque to exceed the transmission rating.
If doing the diesel swap is too much, maybe by cleaning up the aero and going to the numerically low gearing and a T-56 he could slow down the 300 to the point it isn’t such an egregious fuel hog. Mill the head down so his compression ratio goes up and install a more modern ignition that backs off spark advance when it hears knock. Feed the thing 91 octane gas.
Maybe Singh groove the head when milling it. These have gotten quite popular with small-block Chevy builders as the Singh groove allegedly enhances low-end torque. I think Johnny Mullet did the Singh grooves to his Metro. Maybe the Mullet-Man would care to weigh in.
Without the diesel swap, I don’t think 30 MPG is in the cards for this truck. Maybe 22-24, tops with the T-56. The 4BT3.9 is one of the more efficient engines out there and the 300 Six is a dated undersquare design with low efficiency. The diesel swap should be good for at least a 33% improvement in MPG over the 300 Six.