I'm also building another stout 450+hp SBC for mine. Forged Shortblock, AFR 195's, Vic Jr, etc... This will be the third 400+hp SBC in front of my stock 1992 Alpha 1. I've never had any problems with mine, knock on wood. It also does not hurt to have a very light 2600lbs hull and it's very efficient with a good pad.
I recently picked up a spare just in case and am building it up with some good parts.
Mercruiser 1.50 Heavy Duty Upper Gears
All new bearings, vert shaft, prop shaft, clutch dog. I went with a GLM lower gear set as they claim their gears are stronger than OEM Merc's. GLM decreases the number of teeth on the gears making for a larger stronger contact patch as compared to stock.
3 things to keep the Alpha healthy with high hp;
1. Never any WOT hole shots
2. Get off the gas if you get the prop out of the water
3. Deaccelerate slowly, never pull the throttle back fast after going WOT.
Turning the 4blade will make it harder on the Alpha compared to a 3 blade. Proping the boat to get the WOT rpms out of your peak torque range will also help.
I used to run a forged 355 with the AFR's, ported HP manifolds, Vic Jr, 257/269 solid lifter cam thru the Alpha. With a 21p 3 blade Cleaver it would rev to 6500rpms and 78mph. Nobody believed the Alpha would hold up but it took it in stride. I only babied it out of the hole, once over 30mph I showed it no mercy.
What are the specs on your 355? I would like to compare ours. What cam are you going with? Keep in mind the Alpha does not like idling above 800 rpms so tailor your cam choice around that. My old solid lifter was way too large for the Alpha and it was a pain to keep it idling around 800rpms. I had to tune it rich as could be to get it to idle.
I've got it down to three.....
Comp XM270HR 218/224 .495/.503 112
Lunati Voodoo HR 219/227 .515/.530 112LSA
Crane Powermax HR 222/230 .515/.528 112LSA