I think you have an s/s prop, 143/4 dia. with a 21pitch. I think your problem is you have way to much prop on there for that engine and load. I suggest you try less pitch, something like a 141/2 x 19 or even less. You want the engine to be able to get to max revs. A good prop shop can help you out and maybe even loan you a test prop to find the right one.
I would suggest you run the boat and get us the results.Prop size,(usually under the prop nut or on the barrel)max rpm and speed (gps is best) hp of the 350,and the load you test ran with.Then we can make a better suggestion if needed.