Years ago I had a 16' runabout with a volvo outdrive and a 283 Chev. It would pull hard at high speed. I found that there was a small skeg right above the prop that was adjustable. I turned it a little and tried it, helped, I eventually got the boat to run pretty straight by just tuning this way.
Can't swear this will work for you but it solved the prob on my old boat.