Performance mixed with fuel economy!?

66Valiant528 said

A 318 gets way better milage than a 360, but the key is the transmission. An OD 42RH or a 46RH will help get you the highway mpg..2wd mid 80s dodge vans.....a 3.9 would have the 42rh a 360 the 46RH 318 either have to check. A gear vendors od might be a less expensive solution ultimately.

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I disagree about the gear vendor OD.
But an OD trans is the way to go.

@Steve Cotter
No matter the displacement, a dual plane with a mild cam will be very good. A free flowing exhaust and a good intake track so it all breathe easy.

I found that a good working multi spark ignition helps very with mileage as well.