I had a bone stock '71 440 with measured 9.5:1 compression, stock rods, pistons, heads, valves intake, crank etc.

It did have a 750 vacuum secondary 3310 Holley, 1 7/8 headers and a MP .528 mechanical cam. It put out 318-320 HP at the rear wheels and right at 400 HP at the flywheel.

Keeping that in mind I would say the 350, 375 and 390 HP gross factory ratings were dead on!