Had a 351 in a 1988 Supra Pirata. Same issue. The dealer I had purchased it from had it all tuned when I purchased it. It had a 13x13 prop. It ran like that for 2 years. I replaced the engine with a 454 and had new stringers put in it and the old foam removed. Was told that the old foam probably weighed near 500 lbs. It was replaced with 35 lbs of foam. The only thing I could surmise was that all that weight was holding it back. Right now it runs 4200and 42 mph according to the gentleman that worked on it. I will be getting it wet this weekend.

Century63