anyone know the specs of a 500hp.cam,compression,heads,intake,carb,not sure if it's 454 or 502.

500HP is a 502/465HP. Has a hydraulic roller cam, dart intake, roller rockers, rect heads, and a holley 800 or 850 carb.

You can try to duplicate the parts combos, go to roller cams, etc. Or stroke your mag to a 496, a little headwork, roller cam, good intake, holley 800, and make 525-550hp out of a 454.

thunder what do you think the costs are?looking for good reliable 500 hp thats why i was thinking of duplicating the 500hp.i like your idea better but i need to use 87 gas and don't want to have to tune/work on the engine all summer.

If your mag has low hours, then keep the shortblock the way it is. Carb or efi?

If its carbed with a hyf flat tappet cam, (365HP version), you can install a bigger cam, like the 454/420 crane cam, roller rockers, a edelbrock RPM air gap intake, holley 800 double pumper, and some good exhaust manifolds.

Cam=200
Rockers=250
carb=500
intake=250
exhaust 1200

For around 2g, doing the work yourself, you can be in the 420-430HP range. Or go with a hyd roller cam kit, and be in the 450 range, for about 700 bucks more.

Assuming you have the 25 active thunder with the 454 mag, the above mods should really wake up the boat. The 454 mag carbs responded well to cam, carb, exhaust upgrades. You would be happy.

The HP500 carb and 500EFI engines are both 502cube engines. Merc did have a 450HP/454 that used the same cam as the 500HP.
You can build up your 454mag to make 500HP at around 5600rpm.
Port the heads, a cam from RMBuilder, single plan intake, 800CFM carb, and good exhaust with the water entering right at the transom.

Add lifters to that for 500 bucks.

it is the 454 mag carb.what do you guys think i could pick up for speed from 365 hp to 500 hp.would like to run mid 70.running 67-68 now

I think you would gain between 5-8mph.
As an example:
1995 24' Super with a 350HP/454mag carb ran 73mph at 5000rpm spinning a 25p M+.
The engine was pulled and built to make 498HP at 5600rpm on the dyno.
With the upgraded power, the boat ran 82mph at 5600rpm spinning the same prop.
This boat gained 9mph but is a little smaller and lighter than yours...

I'd buy some good exhaust to swap out the cast iron crap first.
Run for the summer and do your upgrades during the winter.

The Merc 454cid/365hp engine has all the good forged internals and has the GM rectangle port heads. Check the heads for cracks between the seats and then have a very mild clean-up/pocket port job done to them i.e. sharp edges/casting marks, etc...and pay special attention to the short turn/side radius'. The first thing you are going to need to do on your mods list is replace the stock Merc exhaust system with a better breathing aftermarket exhaust system like Stainless Marine, GIL, or something comparable---especially if you're looking for 500hp out of a 454.

Roller rocker arms
ARP rod bolts
Hydraulic roller cam/lifters that make power to 5600-5700rpm
Single plane intake manifold (Dart or something comparable)
Holley 750cfm carb
Rings, bearings, gaskets

Most important....Competent precision machine work from a shop that is familiar with and KNOWS what it takes to machine an engine to live and destined for the hi-perf marine environment. Also, have RMBuilder/Bob Madara of Marine Kinetics engineer your cam for you and your engine application. He has done a lot of project cams apps for guys on here. Bob's number is 585-654-8583. Have fun.