I'm kind of a slant fan, but for a truck, I'd probably swap a '94-01 magnum 318 in there with a 600 cfm carb and never look back.....you'll probably get as good of mileage at highway speeds, and more than double your HP (my duster's slant was rated at 101HP net in 1976)

1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)