Sorry to spoil the party, but you are missing some important isues here :
1-A 280 sib is relatively small has too much hull drag, worst trying to plane with more passengers. One thing is to push, move sib, other to full plane.
2-It's s not a go for a short, large pitch issue or call the distributer as this test must be done under trial with an induction tach previously installed to check if inside safe working parameters.
3-Buy a Tiny Tach which is spark programable an set it to fire 2 spark per revolution. The rpm working factory parameters for that engine which is only 170 CC is 5000 to 6000. Stay with the prop that will give slight passing 5500 rpm, will have better hole shot than placing a prop that will give near 6 K rpm nullifying hole shot.
4-Assume you have a well inflated sib to 3.5 psi, tubes and keel, well trimed, correct transom/engine seated and passenger ballanced, correct ?
5-If nothing works out as expected including family dissapointing, you need definitely a larger sib in the 360-380 range, will work better if keeping same engine as hull will drag less and sib will slide better.
Post your pitch/rpm findings if going for a tach.