Either way, I would think you combination is off. If you do not want to get into the engine write now, try a 4 blade prop, see if you can get your slip number down. If you have a 502 with 415hp, I think you will want a smaller carb, dual plane manifold and a 24" to 26" prop. I would test as many 4 blade props in that range as possible.

1.5 ratio, 26"prop, 4700rpm = 77mph - 14%slip = 66mph
1.5 ratio, 24"prop, 5000rpm = 76mph - 14%slip = 65mph

The reason you are not making power up top is your intake and carb do not match the cam and heads. I just hope you a running a cam that will work for a marine engine. I looked up the More Power website. I do not see anything about Marine Engines. Setting up a marine engine is different from a car or truck.
http://morepowerinc.com/pages/812004/index.htm
Just a question, but is the 950 cfm carb built for marine use? I don't think Holly makes a 950cfm marine carb, at least not off the shelf.

"They did not replace cam and when I called to see what it was they did not know what cam was in the engine from when they did the repair."

How did they determine the engine needed a Single Plane manifold and a 950cfm carb?

They did not replace the manifolds, they used the ones that were already on the engine. The only parts they put on were the carb, intake, & Dist.

I sent you a private message. Give me a call.
Brian

I agree with a lot of what everybody said about being over carbed and having the wrong prop.

If you are in the 415-450 hp range with a 5600-5700# dry weight, you speeds are about what I would expect.

For comparison, a 28 Pantera with 500hp, 5200# dry weight, spins a 24" Bravo 1 prop to about 5000rpm and will run upper 60's. You have less HP and more weight.