Only gear that was offered was a 3.91.
Special spread bore carb $$$$$$$$$$
It really was a very special engine!!
At the heart of the Boss's heavily massaged 351 Cleveland was a crankshaft cast of high nodular iron, specially tested for hardness, and held in place by four-bolt mains. Connecting rods were forged, shot-peened for strength and magnafluxed for cracks, and held to the crank by exceptionally strong 3/8-inch bolts. At the other end of the connecting rods were eight pop-up, forged aluminum pistons with three rings apiece. The free-flowing 4V heads were nearly identical to those used on the earlier Boss 302, except for their revised cooling passages. Generous, smoothly contoured intake passages led to the large, canted valves--2.19 inches for the intakes, 1.71 inches for the exhausts--helping the engine breathe freely, allowing "excellent performance potential," as the brochure said. The polyangular combustion chambers created a relatively high compression ratio of a high 11.7:1. Hardened and ground pushrods, solid lifters and special valve springs and keepers beefed up the valvetrain. Like other members of the Cleveland family, the HO 351 had a bore and stroke of 4.00 inches x 3.50 inches.
A cast aluminum over/under intake manifold was capped with a 4-bbl. Autolite model 4300-D carburetor with a spread bore pattern with small primary bores and oversized secondaries. These carburetors were built just for the Boss 351 engine, and are "extremely rare," according to Prochot. "If you can find one, it's probably going to cost you upwards of $3,000 for an unrestored one," said Carol Jennings, business manager for Pony Carburetors, a Las Cruces, New Mexico supplier of carburetors, carburetor parts and restoration services. "And you're probably going to be waiting a long time." The 4300-D draws breath through the Dual Ram Induction hood with its vacuum-actuated ducts. A dual-point, dual-diaphragm distributor fired the Autolite ARF 42 plugs, and cast, finned aluminum covers with special Boss 351 insignias sat over the rocker arms. Ford advised that buyers who wanted an oil cooler could adapt the oil cooler from a 1970 Boss 302; none were installed on the HO 351 at the factory.
