Very impressed with your little squirt boat. Do I understand correctly that you'll power it with a single-cylinder 390, like for a gokart? What kind of pump could you use for that?
Mine was a 23' Spectra hull, bare inside but for some green Astroturf and a stereo, with a Lincoln 460 cu. in. monster under a Holley 751 double-pumper, direct-coupled to a Berkeley pump. Raw exhaust, not a wet exhaust or even dry marine headers, just straight off the manifolds, poked through the transom. Neighbors hated me, so did a lot of the people on Pottsburg Creek and Arlington River. Named it the "Swamp Rocket", 60mph in a foot of water.
Word of experience: Make your tunnel intake ridiculously oversized, with a tight set of steel bars. Mine was 2' long and about 8" wide, and I tried to launch it from a standstill in 6' of water. Sucked up oyster shells from the bottom, jammed in my bars, broke up the vortex too badly to even get it up on plane. Overboard without even a mask, using a 14" screwdriver to pry out the shells.