All good points.

I think you have me talked out of it.

I ran a 87 veleocity with a Alpha drive and 454 magnum 330HP that was massaged with no reliability issues on gears. Boat was 2800 lbs. Without nose cone speed is limited with stub nose on that drive. With cone and 23 Pitch ran 71 mph. Definitely had that boat in the air 8 or nine feet before. Clutch dog wear is problem make sure you use srping loaded shift assist came out in 1989 or jam it in. Easy shifting will wear clutch dog. Also ran a land and sea torque shift prop and that boat literally jumped out of the water. Taught a few lessons to jet boats on acceleration. Top end stayed at 67 with torque shift prop but you could out accelerate most cars .
Also had a friend who put 400 hours on 86 24 foot 4 Winns Liberator with 454 1.36 Alpha and 19 pitch.

I have a friend with a 1987 24' Formula 454mag/330HP hooked to an alpha 1.32 ratio. The drive lasted over four hundred hours before blowing apart. So you can run a a low output 454 on an alpha for a while.
I like the idea of building up the 350 better though...
Good Luck!

on a 2800 lb boat the drive is no problem, my buddy runs a 330 hp with alpha in his 26 searay 8000 lbs. and it takes it running 40 mph.
I think MKast spelled it out pretty clear it is very easily possible, and alot of the brackets, exhaust etc are available from 2nd hand suppliers cheap.
The only question is the weight, liberator put out a 20 ft 330 alpha it was okay. but a SB built would be good just more expensive by the time you get it built properly.

ARE you shure that is not a 305...
The old Merc 188 HP was a FORD 302
The old merc 198 HP was a Chevy 305

Originally it was a 305, and that motor gave way before he bought it. it was replaced with a 350. but the old 198's were 305's

EMI Thunder manifolds, an Edelbrock Performer RPM intake, some tweaking on the Q-Jet, and optimizing your timing will give you some small gains. If you want to seriously wake up that 350, a cam change will usually require you to change to either Vortec heads, or go with World Products Torquer SR or Sportsmans. I prefer Isky Cams, but Comp Cams makes a number of good marine grinds as well. I would not bother without the head change, however. The stock heads do not flow well enough.
I think you will destroy the handling qualities of your boat if you install a BBC in there; you'll be much better off building a good mouse. Definitely pick up a copy of Dennis Moore's book on building the marine SBC- lots of good info in there.
Jim

I took a 1992 Larson senza 25'4" stepped hull boat
running 49mph with a 350 mag 270hp crank, 250 prop hp AND houston prop worked the Laser II prop
225.00 ,I put a set Of EMI thunders Used 350.00 "good shape one year old" Corsa Quick & quiet used "like new" 325.00 plus a new harness 120.00, K & N air cleaner with a snub stack, Plus I reset the timing.
my Wot is now 54-55 depending on how many people are on board

so I spent very little and I have thur hulls which sound faster

5-6 mph gain @ WOT
3-4 in the mid range RPMS

with a boat this sized it should have had a BB from the factory, But this is pretty good for what HP its getting

Seriously... the 454 is only 100 - 150 lbs heavier than the 350, so go with aluminum heads and intake on the 454... same ol, same ol... except it will wake that boat up and run like a raped ape... The way to know if you REALLY want to do this is to ask yourself this question

:Do ya wanna FISH, or CUT BAIT? lol

Make the 350 into a 383. Big block #'s small block package.