425 HP in a Seacraft 20? The Potter 20s will do 55+ with 225 HP. 425 HP just sounds like a creative way to commit suicide. I've touched 50 mph in my 20, but that's all she's got with a 200. But I understand Seacraft hulls start to chine walk at around 60. Here's a post from Fr. Frank I found from about a year and a half ago:
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...Didn't think economy at cruise was an issue to a dude set on running over 60mph  On the other hand you can do more often if there's more gas left to burn 
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You have a point, here. But SeaCrafts CAN be made to run over 60, and I have seen a 20' SeaCraft with a slightly modified hull approach (but not quite reach) 70 mph.
I didn't do the hull work, but I did tune the rig on a 20' SF with a 240 hp. Merc Bridgeport on a hydraulic jackplate. We ended up running a custom prop from 2nd Effort in Lk. Hamilton, FL. The boat was REALLY, REALLY Squirrelly from about 55 to 62-63, and then settled down until reaching 68, which was WOT. Without trim tabs and the hull mod, there is no way it could have been pushed past 62.
The hull mod was done by Rybovich/Spencer in WPB, and included a flat planing pad on the rearmost 5'-6' of the center V, much like the early Steve Stepp 22' Velocity hulls. With a similar modification, I believe a 20' or 23' can easily be pushed past 70 mph with modern outboards.
I think your hull, Steve, can be pushed to near 80 mph with a similar mod, and a change in drive and engine, ..say to Mercruiser EU662-SCI.
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So you may want to consider some hull mods before you stick that rabid small block in a 20.
Dave