seen a 4.3 v6 chev in an aluminum 18' jet boat, 200 hp, 235 lb/ft, spinning a c cut impeller at 4500+/- rpm, 40 mph boat

cheap? = vortec suburban motor, 4bbl, headers, A imp, 18' = 55mph, 16' possibly 60 mph?
or go with jet ski motor(s) and pump(s)
Ralph always says "good, fast and cheap, pick two"
btw...seen a 16' Tahiti with a BBC in it

Just my $0.02
Daniel
Mash on it