To the OP....what size tires are you running? I suspect you may be overly concerned about the rpm of the motor.

So what if it smokes a little on the highway? That's pretty normal for these old all mechanically injected motors.

I'm thinking the only inexpensive solution to this is larger wheels and or tires.

Though some transfer cases may be able to change ratios too.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.