Sure looks like a Merc 40H (Q length)
The motor is a Mark 40 Merc alright. The height of the tower is a "Q" longer length with some "quiet down" built on provisions. The exhaust is supposed to be water level at the bottom big spout. Some one welded on an addition to put the exhaust right into the below water level stream to quiet the motor even more. To make it more original cut that downward snout off to where it is supposed to be water level to perform better and sound better too. Also where the top spout is, is just supposed to be a very short brass outpipe just to relieve exhaust pressure when the engine starts in the water, the main spout takes over once your off.
Some one lengthened the skeg a bit to make it more 55Hish and that gives it better control on a hydro. Stripping the paint off the prop, with sharpening its edges and polishing it will sure improve its performance though it might not be much of a hydro prop. It could be a silk purse from a sow's ear!
Of the 2 Merc 40Hs I had one on 40H on a "H" short tower with opened exhaust plate on the block and Hubbel reed valves installed and we ran it on a Ben Hur hydro back in the early 1970s and she went real well. Not one spill in her day and she ran in the mid 60s miles per hour and she sounded just Grreeeaaat!!!!! Kinda deafened your left side a bit though and upset some neighbours but she was a racing engine!!! :)
Not a genuine Merc Q tower for sure.
I took a few looks at Q towers from pics and that one looks kind of a homebuilt version at a Q length it seems. My hats off to the builder and an "A " for effort in trying to do something similar. That takes some imagination, lots of effort and some knowhow. :)
Why the downer? Its still a Merc Mark 40 Racing Engine!
That Mark 40 Merc motor is mean't to be cleaned up nicely and run on a nice hydro. Its NO mongrel!!! They were the runners of their day and readers around here them. Where they are deficient people find the correct parts over time, restore and run them. There is help out there with possible parts leads to help. Nothing wrong with building the hydro and restoring the engine at the same time and running it, proving out what you are doing.
On some threads here on BRF a reader rebuilt a Merc 40H to a 44 cubic inch displacement, modernized the combustion chambers, with Tillotson KC6 large bore carb upgrades and with all that plus trying to make it look the year it is based on goes 80+ miles per hour and that is just its infancy in coming back. Now that is mixing up the old with the new and you don't have to go that far to have fun and enjoying what your doing. Don't sideline it with too much thought, get on with a Wetback or Ben Hur for her and get at it! :)
Seen a lot of them welded tand braced o the extreme.
KG9 and Mark 40 towers were known to be thin and at times the aluminum brittle. Seen and had so many of these towers crack welded, braced and welded from 3 sides to the extreme and were still proudly used. Comparing to some towers seen that one is a good start to something not bad at all. It depends what your use to. I see a silk purse out of a sow's ear that would dishearten most only to work on it and turn it into that silk purse. I encourage even in the face of adversity. Its percevearance. :)