A buddy of mine has a 29 velocity with a 496 it is a 08 boat.It gps top speed is 67 mph.How fast do you think it will run if he puts a supercharger on it.He is going to put it on the stock motor which is like 425 hp

How much boost ??

6 to 8 pounds

80

with no intercoolers
6# 74-76
8# 78-80

How fast with intercoolers?

ever degree dropped in intake air should be worth 1HP or about a 10% gain. The blower takes power to turn so its kind of a trade off. You could increase boost with the intercooler safely. Speed gain would depend on changes made to overdrive and timing.
Im thinking a extra 2-4 mph depending on which changes were made.

You can't put 6-8 #'s of boost to a 496. The pistons won't last.

I thought the 425 h.p. 496 (mag) had forged internals and 375 h.p. did not???

He didn't ask if the engines would last.