Presumably you have a 25" transom, but I think 500lbs motor on a 20 is bad juju.
Close your transom or stay out of the big pool. Clearly you're hellbent on top speed. In that case get an old 2.4L merc (360 lbs) and put some 225 carbs on it. With gas moving near $5/gal, depending on your budget and use, I'd seriously consider a 140 Zuke or a 115 - 150 etec or optimax or even a 4S Yamaha F115.
my .02