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A vacuum gauge will be your best friend here. I had a '79 F100 with an inline 300 many moons ago with a 600 holley and headers that got 10 mpg period no matter what. 2.75 gear with a 3 on the tree and it cruised at 65 with 10" of vac. Switched the gears to 3.50's and no other changes it got 16-18 mpg and cruised with 18" of vac at 65 and was spinning 1000 rpm faster.

If you are staying N/A with this and putting in that 518 trans I would just advance the stock cam 4 deg and put in a set of 3.55's maybe 3.73's. That will lower the peak torque rpm and it should help get all that mass moving around town. It should do better than most pick-up trucks for mileage on the highway because the aero should be better and the weight is similar.

Kevin

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