Some more info from Wikipedia that just further leads to my confusion.
455
For 1970, the 428 bore was expanded .030" to 4.155 inches (105.5 mm), combined with a 4 7⁄32 (107.16mm) stroke, yielding a total displacement of 457.6 cu in (7.499 L). Oldsmobile and Buick also had '455' inch engines about the same time. For the 1970 model year variants of the engine became available on all full-size Pontiacs, the Grand Prix and for the first time as the 455 HO in the Pontiac GTO, as GM lifted its restrictions on the use of engines larger than 400 cubic inches (401 in some Buicks) (455's in some Olds 442's from 1968) in mid-sized cars. The Pontiac V8 design differs from most other manufacturers' designs in that the external dimensions of each engine, from 326 - 455 cu in displacement, is identical (AMCs 290-401 engines are also externally identical). The displacement is determined internally with changes to the bore and stroke; therefore, there is no "small block" Pontiac engine. The 455 was used through 1976.
The 455, with its "undersquare" dimensions (long stroke relative to bore), emphasized torque over hp, and though advertised as less powerful than some high-performance iterations of the 400, it had a torque rating of 500 ft/lbs., 55 more ft/lbs. of torque than the 1970 performance 400's. The horsepower ratings of this era were often dubious, with engines rated higher or lower in output for advertising, political, or insurance purposes. Per Pontiac's sales brochure, the 1970 455, for example, had similar parts to the higher rated Ram Air 366 HP 400 cu. in., including the same 288/302 camshaft (manual trans. 455) yet was only rated at 360 HP. The 1970 Grand Prix with the same spec. 455 was rated at 370 HP. For 1971 Pontiac introduced another High Output, H.O., version with standard internal parts, a reinforced block with 4-bolt main bearing caps, and improved cylinder head design with 1/8 inch taller intake ports and special round exhaust ports for better breathing, making some 335 hp (250 kW)/224 kW (310 hp in the more accurate SAE net system), but this was an extremely rare engine (it was standard in the Firebird Trans Am. In 1973, a further refined and even stronger version, the Super Duty (SD) engine was introduced with "only" 310 hp (231 kW)/231 kW (SAE net) using a similar camshaft specifications to the Ram Air IV 400. The 455 SD used round port cylinder heads similar to those used on the 1971 and 1972 455 HO, with specific "LS2" intake and cast iron exhaust header-manifolds. Still, it was the strongest American engine offered that year. Its power was achieved through bending of EPA emissions-testing procedures, which led engineers to de-tune the engine to 290 hp (220 kW) via a camshaft change to the same profile used in the early RAIII 400 engines for mid 1973 and 1974, after which point it was discontinued.
While an evolution of the RAIV and H.O. engine designs, the 455 SD was a much improved engine. In addition to the more refined cylinder heads, block casting reinforcements in the lifter galley and main bearing oil pan rail area along with the addition of forged connecting rods with larger 7⁄16-inch-diameter (11 mm) bolts, the SD was made with a provision for dry sump oiling from the factory. This truly was a racing engine, detuned for use in passenger cars.
SD455
Available only in the 1973 and 1974 Formula Firebird and Firebird Trans AM, the SD-455 consisted of a strengthened cylinder block that included 4 bolt main bearings and additional material in various locations for improved strength. Original plans called for a forged crankshaft, although actual production SD455s received nodular iron crankshafts with minor enhancements. Forged rods and forged aluminum pistons were specified, as were unique high flow cylinder heads. A camshaft with 301/313 degrees of advertised duration, 0.407 inch net valve lift, and 76 degrees of valve overlap was specified for actual production engines in lieu of the significantly more aggressive RAM AIR IV style cam that had originally been planned for the engine (initially rated at 310 hp (230 kW) with that cam), but ultimately proved incapable of meeting the tightening emissions standards of the era. The very modest cam, combined with a low compression ratio of 8.4 (advertised) and 7.9:1 actual resulted in 290 SAE NET HP. The initial press cars that were given to the various enthusiast magazines (e.g. HOT ROD and CAR AND DRIVER) were fitted with the RAM AIR IV style cam and functional hoodscoops - a fact that has been confirmed by several Pontiac sources. Actual production test cars ran considerably slower and yielded 1/4 miles times in the 14.5 second/98 MPH range in showroom tune - results that are quite consistent for a car with a curb weight of 3,850 pounds and the rated 290 SAE NET HP figure that some sources suggest was "under-rated." Various Pontiac sources have emphatically stated that NO 310 hp (230 kW) versions of the SD455 were installed in regular production cars. 1975 Factory Service Manual lists the SD455, but the SD455 did not meet emissions for the 1975 model year and was canceled.
455 HO
The 455 HO designation made its debut in 1970 as appeared on vehicle invoice, rated at 500 ft/lbs of torque and 360 or 370 horsepower depending on the body style. 455 HO's were seen in the 1971 model year with HD blocks, The Round port Heads came as #197 for 1971 year and #7F6 for 1972 year. Both having Round Port Exhaust. Special large valve heads with screw in rocker arm studs, Special aluminum intake with removable exhaust crossover, Special streamlined Ram Air Exhaust manifolds, Higher lift and larger duration camshaft and 800 CFM Quadrajet carburetor with specific jetting.
The 455 HO's were similar to the yet to come SD455 in 1973/74. The 455 HO was available in the Firebird (base, Formula and Trans Am), and the LeMans, GTO, T37 and GT37 models. The SD455 took the HO 455 to the next level in durability, power and performance.
Last seen in 1976, ending the era of the 455 HO in lackluster fashion, delivering only a paltry 200 hp (150 kW). It shared nothing with its 1971 brethren except the displacement. Standard in Catalina/ Bonneville Wagons. Option in LeMans, Grand LeMans, Firebird Trans Am, Catalina, Bonneville and Grand Prix.
