This is one of the better battleship kitbashes I have seen. I do have some ideas/suggestions
I suspect the oto melara turret used would be the 'old, round' compact, as the ship is not stealthy anyways.
I would swap some SPG-62's for an director also usable for gun guidance, such as an SPG-60. Also, keep an eye on the angles of the aft SPY-1 radar (although it seems to be alright, on first glance, compared to the burke)
I do wonder about the helideck and the old hangars, if those should not be differently remodelled then what is done now. currently the hangar is only an vulnerability, as is the helideck in its current layout. could an normal helideck not fit on the hull as is? could it not stay on the old deck level, if there is no hangar so no helicopter standard on board blocking the guns? or even, would there not be some kind of helicopter hangar on board, protecting the helicopter and allowing maintenance.
Speaking of remodeling, you have the kitbashing part done great, but now comes the hard work getting everything to fit. for example, the bridge structure taken from the USN battleships does not really fit the higher superstructure. In addition the powerplant would not be gas turbines, right? so the funnel should reflect this.
Based on yours and heuhen's suggestions I've replaced the 76mm OTOs with the older round turrets. Turns out that the older round version is what the surface warships of the JMSDF use anyway.
For some reason I had this impression that the SPG-62 could provide gun control, but now that I'm actually looking into it I honestly have no idea where this impression even came from. Would FCS-2-31s work as replacement for some of the SPG-62s? It's a Japanese designed FCS that (if google translate is not screwing with me) should be capable of directing both guns and missiles.
For the helodeck, my idea was to use a large part of Yamato's historically massive hangar for the VLS, and the minority that remained to stow the two helicopters. Then, have the actual platform be an extension of her stern and have an elevator of some sort to bring the helicopters up from the the hangar.
Something along these general lines (turns out the original image of the model is from Erusia Force, haven't I stolen enough from him yet?):
Nowhere near exact dimensions or executions but you get the general gist of it.
The bridge structure was because I felt the old exposed armored conning tower of the Yamato looked out of place. The new enclosed bridge is really just a lightweight structure built around the original conning tower. USS Iowa as built also had an exposed conning tower with open bridge area and the new bridge was built around it during one of her refits during the war, so I fail to see why it doesn't fit.
Eight (!) LM2500 are specified in the OP, although I am skeptical about how well this would work. Practically speaking, there's no real value (IMHO) to 27 knot speed; this is much faster than a 'phib, but well short of modern fleet speed. Meanwhile, I don't think there is space to enlarge the screws sufficiently to take the ~250kshp necessary to push the ship to the 30.75 knots specified. I'd probably attempt some sort of diesel-electric setup aimed at 20 knots. Springsharp guesstimates this should take about 50kshp, which would allow for a very tidy arrangement of generators. I'd look to the Navweaps forum
for more detail here.
I agree with you about the lack of value of 27-knot speed, which is why I decided to change the engines in the first place. It's actually one of the more implausible parts of this design. One of the main problems is that Yamato's armored citadel is an integral part of her hull structure, including the 200mm armored deck. This meant that adding and replacing a large engine would be nearly impossible (one of the reasons they went for boilers and steam turbines instead of the planned mixed diesel/turbine propulsion). So I decided to add more smaller engines, and decided on the LM2500 which has already seen extensive service with both the USN and JMSDF. The original ref image also seems to mention COGAG, implying gas turbines to begin with.
The real point was to have some amount of increased propulsive power that would allow her to exceed 30 knots and thus have a chance at working with a modern fleet. The 30.75 knots was really just an eyeballed number with no real math or science to back it up as it doesn't really have a real effect on the drawing as a whole outside of the new funnel.