Have a Velocity 260 454 running over 425 hp with a 26p bravo prop 4 blade. Will hit 70 under the right conditions. will run 65 anytime. was turning 5000 at 70. If I get a labbed prop what should I exspect? What pitch should I get?

Labbing will give you 100-300 rpm depending on the recipe (could be more but efficiency can be compromised).
Since you are at 5000 rpm it would make sense that you will need to up pitch by 1-2 inches.
The gains will most likely be in the 2-3 mph range.

Call me if I can answer any other questions.

Brett

Thanks Brett