I have seen what you are describing on some aluminum "Jon" style boats with larger 4 strokes on them.
I addressed the issue on my boat with stoppers for the floor drains (just remember to pull them when it rains).
I have a 20" transom and have never had a wave come into the boat from the rear, into the splashwell yes, but not all the way into the boat.
I would think if you move some weight forward and use stoppers, the extra floatation would not be needed.
But, if you do add floatation, you only need to make up for about 220 to 250 pounds, 630# (your motor) minus say 410# (my motor) or 380# (old school 2 smoke).
Just my $.02.
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Stay Safe!
Sold - 1984 V-20 Cuddy with a 2003 Johnson 140 hp gas sippin 4-stroke.
1995 Ranger 250C with a 2015 Suzuki 300 hp 4-stroke.
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