The XP/GT 150 hp engines were the hot rod bass engines of their day. They put out about 10% more than rated, or about 165 propshaft hp. The 200 engines became down rated to 185 in 1985 when the ratings changed from crank hp to prop hp ratings. So, you are within 20 hp right now- just the way your engine is. The difference between yours and an older 200 would be the higher compression heads, the rubber intake manifold filler blocks (NLA, but worth 5-8 hp.) The carbs are the same (1 5/16" throats, but the 200/185 used larger high speed jets. (.060 jets instead of the .0585 in the 150XP/GT) One other big difference. The 150 blocks (and the small bore flatback 175's) were all considered "low port" engines. The 200 flatback blocks were actually machined to the 235 "high port" spec. The 235 porting let the 200's make more hp at a higher rpm than the low port blocks. That's something you just can't easily change on your block. You will get more hp out of your 150 by: swapping out the heads and the 6 rubber intake filler blocks (322722) from the 200. I am not sure I'd swap the carbs. The old carbs had choke butterfly plates on only 2 of the 3 carbs. I'd run your later XP carbs with the primer choke system and jet the mains up somewhat to compensate for the higher compression heads. Also, your later carbs have mid jets to improve the midrange pickup (not on the old 200 carbs)
