I have a 34' Nor Tech 3-350 Verardo's port outdrive turns counter clockwise center and starboard turns clockwise my prop guy said if I changed the port drive to starboard and the starboard to port I would pickup 3to4 mph.Is this true, would it heart the handling?

Spinning props "out" (port CCW, stbd CW) typically lifts the stern of the boat.
Spinning props "in" (opposite) typically lifts the bow of the boat.

It gets more complicated from here - hulls that have good bow lift with minimal positive drive trim usually see more speed spinning props out.
Hulls that require a lot of positive trim to get the boat running freely usually see more speed spinning props in.
Drive height, though, can minimize or reverse this equation. A 2" higher drive height may like the props spinning opposite vs a lower drive height.
Prop blade styles can also throw a wrench into the game. With a round ear prop, your boat may run faster with props "out", but with cleavers it may like them "in".

And we haven't mentioned that prop rotation creates a different handling dynamic on some hulls. Around the docks, the boat will respond differently when you reverse prop rotation and you'll have to relearn how much it side torques and crabwalks when spinning and maneuvering by shifting FWD REV.

Best bet is to talk to somebody who has tried all of these combinations on your hull and similar power.

I'm sure if NorTech could have picked up 3-4 mph by spinning props in with no negative side effects, they would have set it up that way.
Spinning in will make docking more difficult.

I want to thank both of you for your input.I don't have a trailer my boat is on a lift at the house I would have to take it to my dealer to do the changes, and if it doesn't work I'd have to change it back.
I would think Nor Tech should know best, but Fountain turns them the other way.

Cant you go to your local sandbar, trim up and swap them?

You have to pull the drives to do the change.