I've read in a few differnet places that the real HP of the old 330 454 is only around 290 crank hp. Is there any truth in this?

The older stuff was rated at the crank and you have to figure a loss thru the drive. I would say this is probably true depending on what year it is. I don't know what year they started rating them at the prop shaft

they used to rate them at the crankshaft at 330 hp,i beleive that was pretty close,then they changed to rating at the propshaft,that,s when it went to 300 hp.i was told merc did that to make insurance easier on the wallet,but i don,t know if that is correct.