lol, gonna throw some BIG ponies at it, huh?

My goodness, my hat is off to you for all the effort you put into this and a never-give-up attitude. Objective evidence indicates that, as you suspect, the hull itself is a limiting factor. Not sure if you have tabs or not, there could possibly be some help there but...man, like I say...great to see you still have a good attitude about all this.

There have been a few threads like this over the years where an owner made a major investment, put significantly more power into the hull, and was just totally devastated by the lack of any real increase in top speed. I have this idea that floats into my head every now and then about dropping in a HP500 EFI in place of my stock 454 MPI, but if I couldn't gain 6 - 8 mph out of the swap, it just wouldn't be worth it.

Keep us posted with your new project!

Your drive is going to be pissed...lol.. But thats how the ball rolls, you upgrade something, then need to upgrade something else to handle the upgrade. But when the gps speedo hits 80 all the pain goes away and the smile gets huge.

Try a 23 pitch Mirage Plus prop. 3 blade

The solution to better understanding engine dynamics, is more and better education. Not keeping people in the dark.

There is plenty of good info online and in performance books on compression ratios. Using the dynamic compression and applying it to a chart of usability is far more effective than just the Static compression.

Chamber style, quench, timing, spark strength AF:R, all play a part in setting up a reliable performance engine.

I like to run a little higher compression, within reason, for the part throttle benefits, where most marine engines are used their entire life.

Just saw this was a resurrected post. I pulled down my reply.

When it comes to a boat, BIGGER cubic inches is always better. You cannot go wrong or backwards with CID, but you obviously have to get the power to the prop.

I didn't read the entire thread, and although it is NOT completely necessary, but putting your engine on a dyno would serve you well so that you can (for the most part) properly tune it and get it in the ballpark of where it needs to be and you would know what you've got for horsepower and torque you have at any given RPM level---it will give you a better idea of where to prop it, etc. If it was mine, that is what I would do. I know dyno testing is an added expense, but it would save you some time, guess work and grief....just my .02

Anyway, I like your project going from a 350cid to 383 for a boat...it's a very interesting challenge. Hopefully, you will have some fun while figuring out all the variables and obstacles to get the results you are looking for..you'll get there.

Full dual plane intake needs bigger carb
 

OP went with a Sniper EFI. Not sure how that turned out.

One of the issues he may have had with the 383 upgrade and a Performer true dual plane (no cutout) is the need for bigger a bigger carb. Each side if 4 cyl on draw from a Primary and Secondary. Vs an open plenum design that feeds each cyl from all 4 carb barrels.

The 383 could simply not able to draw enough air through the carb. Despite the 23p slip, the boat ran very similar speed for a drop to 4200 ish RPM, meaning the engine isn't getting to full potential even at the 5200 with the lower pitch prop.

Any cutout or spacer to put a bandaid on a too small carb with create poor flow between plenums. Solution....gey a bigger carb. 850 Vac Sec fir the healthy 383. That is only 425 CFM to feed a 383 at 6000 with 100% vol eff is pumping 332 per bank of 4 cyl.

KAAMA makes a great point on the dyno. This issue could have been solved before the engine was into the boat, and propping it would be far easier.

Setting up for WOT on the dyno, tuning for cruise in the boat.

Well, Since were still taking about it. My post was about the carb. Stock 350 couldn’t have a 800 cfm carb as stock. Usually a 600 cfm carb.

Am I missing something here does any one think that his cam with 64* closing is a problem? I meant to say on the intake valve.

After you said that, made me think that's late.
Looking up the cam (that appears to be CP number he listed) it's a COMP XM276HR.
All the other specs aren't radical.
That closing is at the advertised duration.
Actually spec says 68°.