I know my Formula 242 has the 454/alpha and i have had it 17 years easy out of the hole and you'll be fine. Go for it.

I use a Mark IV BBC roller stroker motor (496) in front of my Volvo Penta so it can be done with a Alpha. Like Masi said, easy out of the hole and no jerking on the throttle stick. Also change to gear oil with some thing good.

fou,
I have an '87 CC 230 Limited, love the boat, engine lost a valve (540hrs) after pulling the engine I found the bulkhead behind the gas tank that got soft, all else is good. I'm going to pull the tank while I'm at it just to be safe.
I built a 383 with Bob Maderas help, should push the boat nicely. I will be using the alpha also, and I do have a spare, as everyone has said, be easy on it and see how it goes.
Have fun!

I plan on saving some weight by swapping the exhaust manifolds for aluminum ones. It should be about 100lbs less and I'm too far form salt water...

I had a 454 Magnum/Alpha combo in a 1987 268 Sea Ray cabin cruiser, the Alpha had special upper gears in a 1.32 gear ratio and casting mods to the upper housing, made to stand up to the 454 which was 330 hp in 1987.

That original Alpha stood up to the 454 for the next couple of years like a champ (about 150 hrs) until I hit a log, it would do 40 mph no problems and would get on plane easily. I got it rebuilt lasted 5 hours, put on a new one it lasted one season almost (like 40 hours), rebuilt 12 hrs. I put on a 1.5 Gear ratio V8 alpha drive almost got 2 seasons out of it (about 100 hours). Then I swapped to a 7.3L diesel with a Bravo 3, no problems with the outdrive since.

Moral of the story it will work but you will definitely find the shortcomings of the Alpha outdrive in very short order with any hint of abuse. Hit anything underwater, it's scrap. Jump a wave, hello broken lower Pinion gear. Hammer the throttle, dealer's choice upper gear set or lower pinion again.

The shift interrupt from the Alpha will still work. You will have to get big block manifolds so shop them together with the switchable exhaust you are considering.
The distributor is the same, and the ignition control module should work the same. The module can be identified by it's application and advance. Example: it will probably say V8 24, meaning it was installed to your 350 V8 and has 24 degrees advance. This will work in a stock 454, I think they run that advance in Bravo controllers too.

I think the less weight of your boat will allow the Alpha to live longer than LC's 26'8" SeaRay cabin boat.
Masi is a good example.
I am 2 years in to a 383 in front of an Alpha and I never boot it out of the hole. My Formula is the next size smaller than Masi's. It probably has some more horsepower, but it has similar or less torque. My boat weighs less and has 2 degrees less deadrise than his, so I am optimistic for a good amount of service from my drive.
One thing I did when I installed the 383 was to put high quality gear lube in the drive. I used Amsoil Severe Gear. Haven't had problems.

NHGuy is probably right as I had another factory Alpha 454 set up in a 21' Chris craft Scorpion that I got on trade, it lasted 2 seasons that I had it and the guy I sold it to ran it for another 10 years without issue before rebuilding the drive. I did stress to the buyer changing outdrive oil with a good oil (I used Quicksilver HD) every 50 hrs is a very good idea, especially if he was throttle happy. This was mainly to check the oil condition and to make sure the oil level level was correct.

If I still had an Alpha 454 set up, one thing I would probably run is a outdrive shower/cooler. I have never seen seen as much discoloration on the outside of an outdrive upper housing than these 454 Alpha set ups which leads me to believe there was a lot of heat generated in the uppers. It's probably the reason I had to change the oil so much, now I think about it an inboard outdrive gear lube monitor would also be a good idea to keep the gear oil topped up.

I've been told the max horsepower of the Alpha was 300hp, and that the gear change for the BBC was to use a tougher/thicker gear to cover the 330hp needed while they finished development on the Bravo. I ran a 260hp Merc in a 21' walkthru on an Alpha for several years, then built a blower motor (SBC) for it, nothing too crazy, but likely 400+hp. Year one I blew the lower unit gears pulling skiers. Year two I blew the upper box gear set. Year three a guy wanted it a whole lot more than I did, and it went bye bye. Some live under bigger horsepower, some don't for some reason. But my experience myself and friends at that time was if you put fairly decent power through them, they will give it up.

Thinking of how I pull up skiers with full power. I'm not surprised that a supercharger lunched an Alpha.