I found a bit of info on wikipedia about these.....
"1971 R-code (Boss 351)
The Boss 351 was the most potent high-performance variant of the 351C available only in the 1971 Boss 351 Mustang. Rated at 330 bhp (246 kW), 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.[1][13] Forged domed pistons gave an 11.3:1 nominal (11.1:1 advertised) compression ratio which made premium fuel necessary.[12][14] It had four-bolt main bearing caps selected for hardness and a premium cast-iron crankshaft selected for hardness (90% nodularity).[1] 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.[3] The valvetrain used hardened and ground push rods with guide plates, and single grove-hardened valve split locks.[1] The forged connecting rods were shot-peened and magnafluxed for strength, and used improved durability 180,000 PSI 3/8-inch nuts and bolts.[1] 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 1,806 Boss 351 Mustangs in 1971, 591 of which are registered and accounted for on the Boss 351 Registry site.[15]
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 383 hp (286 kW) gross at 6,100 rpm, and 391 lb·ft (530 N·m) torque (gross) at 4,000 rpm.[16] 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.
1972 R-code (351 HO)
The 351C HO "R-code" had a number of changes to help meet emission standards for 1972 compared to the 1971 Boss 351 "R-Code". The camshaft had less duration, but more valve lift, while the mechanical lifters remained unchanged.[1] The forged pistons were changed to flat-top style and the heads to open chamber heads, but retained the same large ports, valves, and adjustable valve train used in 1971.[3] This resulted in a compression ratio decreased to 9.2:1 while the cleaner-burning open-chamber heads helped meet the new emissions regulations.[1] The Ram Air option was no longer available. The engine otherwise remained unchanged from 1971. This engine produced 275 hp (205 kW) using the more realistic SAE net system and was only available in the 1972 Ford Mustang.[3]"
Notes:
[1] Hammill, Des (Sep 2011). Ford Cleveland. Dorset England: Veloce Publishing.
[3] Sessler, Peter (2010). The Ultimate V-8 Engine Data Book (2nd ed.). Minneapolis, MN: Motorbooks International.
[12] 1971 Ford Mustang (Sales Brochure ed.). Ford Motor Company. 1970.
[13] Traver, David (February 2010). "Last of the Hot Rods - 1973 Ford Mustang". Hemmings. Retrieved 3 June 2014.
[14] "351 Cleveland". Boss 351 Registry. Retrieved 3 June 2014.
[15] "Welcome to the Boss 351 Registry". Boss 351 Registry.
[16] "Muscle Car Engine Shootout - Ford Boss 351 Vs. Chevy LT-1 350". Hot Rod Magazine.
[18] Only 398 Ford Mustangs with the 351C HO engine were produced according to production data from Ford
