I wasnt propping for peak torque.
The dyno program I'm using shows a drop of 135 foot pounds from 5000 to 6000 rpms.
With the exhaust.Heads Cam etc all programmed.
If he had better flowing heads and exhaust I don't think it would drop off like it shows.
I know it's not exact but they usually are accurate to show a pattern and when torque drops.

So of you think that he will have a higher top speed with that kinda drop off then i dont know what to tell you.
I personally do not.
Ps with that cam he will have around 580 foot pounds at 2000 rpms so I doubt it would be a turd.

Did you input what intake he's using also. Dual plane, single plane big dif in torque curve. I understand what your asking however you completely lost me when saying he'd still have to prop for 4800-5000 with the 651 cam. More like what he's looking for if I had to guess. 5,700.

Yea I understand. Not everyone likes dry pipes. I've had several nasty comments concerning my exhausts but even more complements and there are places I simply have to avoid.

thats funny. Some people just like to complain because while others dig it. I don't like waking up to fishing boats leaving at 5:00 AM nor sailboats noisy mast clanking all night long either. I don't complain about that. See how they like my twin 900 SC plus dry pipes. Lol... From past experience I have to agree with you though, the higher percenter like it.

Tried both and torque still dropped big time between 5000 and 6000.

You just keep concentrating on the cam, cubic inches and hp numbers.
The torque is what drive the boat to accelerate.
you can have a higher Hp number because you are spinning the engine faster and still be dropping torque like a rock.
If the boat was making 450 lbs torque when it topped out at 5400 then it needs more then 450 to keep accelerating.
Which means you would need to prop it to a point where it had more then 450 or it's not going to go any faster.

The dyno program showed his current cam dropping off from 5000 and being down considerably by 5400.
Just like it is doing in real life.
And the 651 is smaller and for a lower rpm range then his current cam.
So I really don't know how you think it will make the boat have a higher top speed.

After reading it looks like the 651 will revert so I would recommend the 771 cam
or something close in a different brand.

So it seems to me from the information given I need new exhaust if I want to get any kind of RPM and a decent cam. Since I already have it down to a short block I will probably look for heads or have a pair of those promax 317's done. My question on headers is I see a lot of used ones on this site and I really don't think I am going to buy new headers for this boat as they would be more than I paid for the whole thing. Would any name brand header be OK or should I be looking for a specific brand model for my engine size as I have seen some that look to be aimed at a 496?

I think that's an excellent idea.
I don't know near as much about good headers versus bad ones.
So I'm going to bow out.

You might want to start a new topic about quality used headers etc

I'd go with Stainless Marine manifolds with long tailpipes, (all the way through the transom). Water exits at the very end of the pipe. That's what I run except I welded up the water exit holes and welded in a bung near the end of each pipe and dump the water out the side of the hull. They are super reliable, relatively inexpensive compared to headers and no worries with water crawling that distance back up the tailpipe.

We docked at a restaurant back in the summer that had their wine glasses handing in a rack. When we walked in the bartender was like, "dude, your boat is so loud the damn wine glasses were clinking together."