I am trying to find an extension box imco ,a friend of mine told me that with the extension box I also need to change propeller. Is it true ?
At the moment I have a 4 blade 26p. What propeller I need if I do the box ?
boat 26 scarab with 700hp bbc

it depends on where the prop shaft ends up.best to start w/ what you have and see if you need to go up or down in pitch.
not familiar w/ the scarab,but will that hull like a box,small singles can be finicky when a box is added.but if its been done before w/ success,go for it.

You seem way under propped for a true 700 hp........... what rpm are you spinning it ?

Not all boats like ext boxes. No specific reason to change the prop. Only testing will tell.