If you're already around 300 hp and going for more, you probably will need a 9", especially if you drive it hard with the 4 speed manual. Junk yards all have the cross over books telling what other model and year parts will fit a given application. Inventory is usually computerized from one yard to another. They can probably find a used 9" 3.55 Traction-Lock somewhere in your state. The problem with used and unrebuilt is you won't know how much wear it has on it. You might end up having to get it rebuilt in a short time anyway. I bet you can find a suitable used 9" though just by perusing the salvage yards and asking them to look for one.
Rev
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'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.
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