[MrPbody]

400 Pontiac was first introduced in the '67 model year, and produced in quantity through '78. The last year in cars was '79, but most of those were "leftovers" from '78. The factory ratings were anywhere from 265 (2-bbl, low compression) to 370 ("Ram Air IV"). The earlier engines (pre-71) were rated as "Gross Brake Horsepower". That is, power at the flywheel with no accessories.

In '71, rating systems were changed to "SAE Net" horsepower. That is, power at the tailshaft of the transmission with all accessories installed and operating (power steering, alternator, water pump, A/C, etc.). Also in '71, compression ratios dropped significantly to allow the federally-mandated use of "regular" (lower octane) fuel. What this means is, trying to use today's methods of rating power will not accurately describe the power output of those engines.

I replied to your post in the "general" section. Go there for more info. The 400 Pontiac is the second most prolific performance engine of the "muscle car" era, second only to the 350 Chevy, which is NOT a true "muscle car" engine. 396 would be Chevy's "entry level" muscle car engine. From '64 through '70 (the "heyday" of muscle cars), more GTOs were sold than any other model of muscle car, including SS396. SS finally sold more in '70, but was far behind overall.

In today's world, it's a piece of cake to build a 400 Pontiac to make 400-plus horsepower using 93 octane gas. Prudent parts procurement and careful attention to the little things will make it the nicest performer you've ever driven. GTO did not earn it's reputation from losing...

Jim