Some say a Gen 2 can take up to 400 hp ? Is there a way to build a Gen 2 to handle]a 383 stroker with about 450 hp ?
Any hi perf upgrades that can be done to the drive ?

Nope,.... Not really,.... It'll last so long as ya don't beat on it to badly,....
Yer right hand will determine how long it lives,...

Good news is, SEI replacement drives are only 'bout $1,500.00,......

whom told you 400 hp ? They really do not know what they are talking about. OEM MERC states 300 max hp for alpha gen 2.

OEM builds --they do not use it for anything past the 300 hp / 350 V8 HO mag 300 hp

here is what I found online to prove what I said about the 300 hp ---- this company post oem info and specs

Alpha one & gen 2 --- MAX hp rated in the link

https://www.perfprotech.com/blog/articles/alpha-specs#

There are many with small light boats getting years of use out of Alfalfa's with 350-400hp. BUT ! That is with 350's.

383's are used successful too, BUT ! They seem to keep Alfalfa's around if no more than say 425 ft/lbs of torque..to pick a #....Which is very mild 383's. Say mid 300hp level or under.

HP is one thing, torque is another, and most importantly is how light and efficient the boat is. Same thing applies for Bravos, but obviously at a higher power threshold.

but thru oem Merc and real boat builders - matching engines to alpha gen 2 are not going to mate them past 300 hp nor past the 350 hp ho mag MPI 300 hp. That was my point for posting the true OEM spec on this. What one does afterwards is their own biz after the boat is bought new or used. Merc themselves will not warranty past 300 hp. I have actually seen the Merc rep come in personally to inspect this exact situation on warranty for Alpha outdrives. .

I agree,

Well said SB- I run 350's/425hp in a "Historic" boat and have stayed away from 383's..avoid hole shots with a relatively light hull.
Not sure how much of this pertains to the drive in question...

I have:

1) Installed drive showers
2) Installed vertical shafts w/o oring grooves
3) increased bearing pre-load on upper gear box bearing (drive builder's idea)
4) Updated upper gear sets..one less tooth and improved tooth form (I'm told)
5) Converted to wet spline (uper vert shaft) with double seal from single seal/grease set up- not much for strength..maybe less wear long term..

I run thru hull exhaust and routinely get 4 years on H20 pumps