The Boss 351 was the most potent high-performance variant of the 351C available only in the 1971 Boss 351 Mustang. Rated at 330 HP, it was fitted with a four-barrel Autolite model 4300-D spreadbore carburetor, an aluminum intake manifold, solid lifters, dual-point distributor, a six-quart oil pan, and cast-aluminum valve covers. Forged domed pistons gave an 11.3:1 nominal (11.1:1 advertised) compression ratio which made premium fuel necessary. It had four-bolt main bearing caps selected for hardness and a premium cast-iron crankshaft selected for hardness (90% nodularity). The cylinder head was modified for better airflow, used screw-in studs with adjustable rocker arms, and except for the water passages were basically the same heads used on the Boss 302. The valvetrain used hardened and ground push rods with guide plates, and single grove-hardened valve split locks. The forged connecting rods were shot-peened and magnafluxed for strength, and used improved durability 180,000 PSI 3/8-inch nuts and bolts. The R-code Boss 351 Mustang was only installed in the 1971 Boss 351 Mustang, and it came equipped with Ram Air induction. Ford manufactured 1,805 Boss 351 Mustangs in 1971, 590 of which are registered and accounted for on the Boss 351 Registry site.
The January 2010 issue of Hot Rod reported a project in which a Boss 351 was assembled to the exact internal specifications of an original motor, but fitted with open, long tube, 1-3/4-inch Hooker headers (vs. the stock cast-iron manifolds), a facility water pump, a 750 Holley Street HP-series carburetor (vs. the stock 715 CFM Autolite unit), and minus the factory air filter assembly, engine accessories, or factory exhaust system. In that mildly modified state, it produced 385 HP gross at 6,100 rpm, and 390 lb·ft torque (gross) at 4,000 rpm. A measurement of SAE net horsepower would be significantly lower, and represents a more realistic as-installed configuration with all engine accessories, air cleaner assembly, and automobile exhaust system.