My vote is a 3.23 rear gear and an M-20. Pontiac sold 326 powered cars with 3.08 gears and the same trans. Your 455 should be able to tough it out. I yanked the 3.36 gears out of my '67 GTO about 15 years ago and swapped in a 2.56 posi......I love cruising 80 mph at 2450 rpm and getting 21 mpg while doing it. My days of screaming along in the slow lane getting passed by Hondas and Subarus getting 10mpg and running hot are far behind me and I have no desire to return to them.
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Jeff 
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