Hey Dan, I'd want to go with at least an 8 if I were you. Not sure of the bow on your new rig, but the wind can wreak havoc on your troll if you're underpowered.
Not to mention how much harder you have to work the engine to push, and then there is always the issue of emergency back up...
5 won't get you far fast, especially with wind and tide against you...
I have an 8 right now, but would ideally go to a 9.9 Bigfoot for that extra push in a ****ty situation.