I already have the merc upgrade cam #431-850478. 224 with .483 lift. I guess that is the 450 hp cam? I do have emi thunders so I don't think reversion would be a problem with a bigger cam. I was thinking the hp500 cam but would that even be worth it? Or would I just go like a 525 cam?

I believe the cam you have is the stock 454 and 502 magnum cam. It should be a roller lifter cam. Thats a fairly mild cam, and upgrading it can be worth some power.

Per engine specs in your other post. Off the shelf hydraulic roller.
http://www.summitracing.com/parts/CRN-139011/

That cam isn't much bigger then the cam I have now.

The duration isn't much different. The duration determines the rpm range the motor will make power in. You increase the duration and the power band will move up into a higher rpm range. That cam has a operating range of 1600-5600 rpm. It does have quite a bit more lift, over .100" or about 20+ percent. You had stated in your other post you didn't want to run it over 5200.
You don't want to go with anymore duration or you will loose power on the lower end. This is a very popular off the shelf cam for marine big blocks. I would say it's the biggest you would ever want to go in your engine.

That cam is about the same as a hp 500 cam. Little more duration and little less lift? Or am I just stupid when it comes to cams?

You are absolutely correct. They are very close to being the same. I never knew the specs of an HP 500 carb cam but after looking it up if I am correct they are 284/292 duration and 576/598 lift. Still the most you would ever want for your engine and quite a bit more lift than what you have now. May want to have your springs checked for the increased lift and deffinately get some roller rockers.

If you wanna go faster, go with a crane 731or 741 cam. Spin the motor to 5500. If you don't want to spin the engine any faster than 5200, the cam Jim recommended will add some power over your stock magnum cam.more duration on the exhaust side will help with stock gm heads crappy exhaust flow.hundreds of guys have used the 731 and 741 cams in their 502 mercs with good results. my buddies 502's made 600hp with the 741 cam and merlin iron heads, which don't flow that much better than stock gm rect ports.

5400-5500 isn't a problem for a big block with forged rotating assemblies , and a beefy valvetrain. I used to think that spinning the motor faster would hurt it. I've learned spinning it to slow and Overpropping it is worse.

The cam I linked to above, Crane 139011 has the same specs as the Crane 731. Difference is it's to retrofit the mark 4 and gen 5. The 731 and 741 are for gen 6 motors.

I assumed he had a gen vi since he had a 502 mag roller cam...