Rather than a car transmission, due to your short wheel base, you might check into New Venture 5-speeds (NV3550 or NV4500). The NV3550 is a little lighter duty, not sure if it'll handle 500HP. The NV4500 was available in the Dodge and Chevy diesels and will withstand your floggings. If you still need something stouter, look at the Clark 5-speed from medium-duty trucks. All the above should have bellhousing adaptors for your 351 Ford. All the above transmissions have had numerous write-ups in many magazines, check into the Advance Adaptors website for some other good information.

Other considerations:
1. With 500HP and 38's, you're going to pop a Chevy 10-bolt with relative ease. Use a Dana 44 or Dana 60, both of which are available as 6-lug rear axles.

2. 50-60 miles/day on your Swampers will kill them in short order.

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