I'm not sure on the mileage difference. I am running a powerglide now and I love it, but I don't know if it would be worth it for you to even swap. If your planning on dropping the th350 then your best upgrade for street use would be an overdrive 700r4.
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1964 Malibu SS, now a LS1 and T56, Back halfed, rollbar, narrowed 9 inch, 4.11 gears and 31x16x15 Mickey SSR's on Weld wheels.
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