If remember correctly riser extension not avail because the MPI has a DRY Joint riser, ie water does not flow between manifold and riser. Check spec for riser height above water line. Ditto recommend looking at "Y" pipe attachment to inner ransom.
While your at it REPLACE the flappers in the "Y" pipe.
Good luck, and hold on for a fast ride.

They are available, at least in the catalog- I have priced them for my dry joint MPI. I think another 25 owner has them?

I also have 1.62 gears in my Merc Alpha I.(Tsunami 23) After some experiment with props of many sizes,I ended up going with a 15'-1/2 x 17 Mirage Stainless steel prop.it pretty much works perfect at all rpm ranges.

When I did my restore on the Tsunami,I had planned on going all new with a new engine and new outdrive.Unfortunately reality in the financials hit home,so I went with a donor boat that just went through an engine with a new long block 350 carbed Mercruiser.The guy sold it as the transom was shot,and didn't want to sink any more money into it.It only had 200 hours on it.

I almost hate saying this because I don't want to jinx it,but after 5 full seasons,it's been pretty much trouble free.Personally,I like the carb,(I used a 600cfm Edelbrock marine carb).I know that in an emergency I can rube goldberg the carb if I had a problem,but not so sure on the MPI,i'd probably be out of my league.
Also in the area I fish,virtually all the boats are either older I/O's or inboards,and the one shop I use is an expert on those systems.He also has all the parts on hand and is very reasonable.

Good luck which ever way you decide to go.

Hey BigEasy. Thanks for the feedback and glad to hear your engine worked out so well for you. I ended up going with a 300 hp 357 MPI and it is great. Tons of power, smooth and quiet. I also need to reprop. I just finished the 20 hour break in so now I can figure out what I need. I currently have 1.62 gears and a 14 1/8 x 19 prop. I was at 40mph at 4200 rpm with a lot of throttle to go.

Glad it all worked out for you,I still would love to have something like you went with attached to a Bravo drive,but for now I'll have to stick with what I have.
I also remember being worried about the 1.62 gears,but Father Frank and the guys over at Prop Gods were a big help in prop selection.
In fact when I posted a thread about what size prop to go with,it turned into a fairly long thread about the advantages of those gears.I remember someone mentioning that Mosely preferred them over the 1.50's and other guys who raced did as well.My WOT is 4600 rpms(at least that's what I was told)for the engine I have.A few weeks ago on a flat day with the trim set just right,I was at about 42 mph(gps) at 4500 rpms.
That was with the canvas up which is like a big sail.I never run at wot,I try to keep it around 3200 to 3400 rpms.
If I can find that old thread I post it up just for the info.

That is good to hear about the gears BigEasy. I was told they are weak and I should change them. For now I am going to go with the prop change. 42 mph is fast! With my old 5.0 I was only able to get to 35 at wot. I am happy with the added top speed but like you I will rarely use it. I think the manual says my wot is 5200 rpm.

Took the boat out in the bay here and through the golden gate. Performed fantastic. Couldn't be happier with the ride and the engine. There were five of us fishing in the boat. Dry ride for everyone.