Mark I'm glad you like your 300 I-6, but if you really want to try a real "Truck" gas engine, you ought to try a 460.
My EFI 460 4X4 5-speed 4.10 gears, gets 12 miles per gallon, has a lot more horsepower and torque the 300 I-6. I pull a 9000 pound 5th wheel trailer with it and your 300 I-6 empty would have trouble keeping up with my 6800 lb truck and 9000 lb trailer...
Having driven 2 Ford 300 I-6 engines (a 1982, carbureted and a 1992 EFI) to over 300,000 miles, and having driven many ford 302, 351, 429, and 460 engines well into the multi 100,000 mile range. The 300 I-6 is a DOG. I have a tractor that will pull stumps, but it tops out at 16 miles per hour...
Unless your truck is competing in some "LOW horsepower" type of tractor pulls, or putting around a field in granny (LOW) gear full time the 300 I-6 has ZERO advantage over the nearly same displacement 302 V-8 (Fords most successful engine ever). The stark difference in on road and highway performance is incredible. The 300 loses any torque advantage it might have above 25 miles per hour... All of my 300 I-6 engines have always gotten less gas mileage than my 302's 12-16 MPG for the truck application 302's
10-12 MPG for the 300's.