Welcome to the forum mate ,
I just add weight to the front of mine , how much all depends on the day , anything from 25-50 kg seems to do me . The reason I add weight is it not worth messing with fins for me as the engine I am using at the moment is only a 5 hp . Remember though adding weight will slow you down .
You have 3 options ...
1: Add some weight .
2: Add a fin to the back of the boat .
3: Add the fins to your engine .
What ever you choose remember getting the boat to plane nice and flat is all well and good , I am not keen on the way this can make the boat nose dive into big waves .
All depends how rough the weather is you fish in ?? With any fins you would still be better with some weight when out in some proper weather as boats can and do flip from time to time .
I like to have just enough weight that I feel the boat will not flip on me from either a wave of the wind+wave catching the underside , but just little enough that the nose is slightly up .
Another thing you need to think about it with fins added there is the thing that when steering the boat will act a hell of a lot different to one that has no fins , it will roll a lot . Also if it is fitted to the back of the boat don't reverse to quickly
At the end of the day it is all about pros and cons , thinking about what you will gain over what you will lose , and what you actually need and want from your boat and engine , and what conditions you will be out in ??
Probably not a lot of help really but there you go .......