Our 375 and then 405 ratings are what used to be called "gross" hp back in the old days of the 60's & early 70's.
The OE's used to rate each type engine without any accessories in a test cell. That was the number they gave to the world at large as how much hp the motor produced. I guess the insurance industry used that number for the risk level of a car???? Anyway there was much skullduggery going on. In 1971 the SAE folks wanted some accessories to be included; by '72 it was agreed that a motor should be tested with all it's accessories in place and with the exhaust manifolds and later CATs in place ( I'm 100% sure about the CATs ) and that hp figure would be called SAE corrected net hp. They were trying to get as close to "as installed" as they could to give a hp figure. I figure that would include W/P, Alt, A.I.R. pump, P/S, exh manifolds, all emissions' controls and programing. Anyway, since 72 all engines are tested in this formula (?) to give apples to apples numbers and no funny stuff.
As an example, since I had one of these, the 70 C3 LT-1 yielded 370 hp @ 4800 rpm; a 71 C3 LT-1 yielded 330 hp @ 4800 rpm; a 72 C3 LT-1 = 255 hp @ same rpm. The only running mechanical changes were larger combustion chamber heads for 71 & 72 vs 70; cams were made a bit milder as time went on proly due to CC size, 64 cc chambers in 70 to 76 cc chambers in 71 & 72. The blocks, pistons & rings & rods and piston top ring to deck measurement at TDC remained the same. Im my 72's owners manual the graphs for hp and T/Q stopped at 4800 rpm unlike our graphs that run out to the real peak rpm points. So I'm guessing that the SAE also eventually required the OE's not play games with the rpm that they used to get a max reading.
Tom