The round-top carbs that you are using now are not ideally suited to the cam you have in the K7/8 engines.
The round-top carbs go slightly lean between 45% and 55% of throttle: you can see this zone by looking down their throat - you will see the wasp-waist in the middle, which was intended to increase the lift on the needle jet at that range to bump up the torque a little in midrange before the cam comes on at 5500 RPM.
The K7/8 has a cam that comes on around 6200 RPM instead of the earlier 5000 RPM of the earlier engine. The PD carbs, while leaner overall, were made richer in the 4000-6000 RPM range to boost its torque there.
When the roundtop carbs are used with the later cams, there is a lean spot between 6000-7000 RPM because of this difference, and the roundtops essentially do not work well above 7500 RPM, in order to protect Honda's warranty department.
This stacks up to a re-tune-needed situation for use in your arrangement. The floats need to be set deeper so as to increase the lifted fuel above 6000 RPM, and the holes in the emulsifiers need to be opened up somewhat to accommodate the increase. I'd recommend drilling out the upper 8 emulsifier holes to about 0.039" (they are about 0.035" or 0.0375" now, depending on which ones you have), and increase the mainjet from the stock #105 (K3) to about #120 or #125. Make the float bowls 24mm deep. If there is a stubborn flat spot at about 7000 RPM, then open up the lower 4 smaller emulsifier holes from their stock 0.0245" to about 0.028" or even 0.030" as the last resort.
When these carbs are run at full speed like this, they MUST have a steady air flow into their float bowls. This is why the expensive bowl vent parts were installed: on the K0 bikes there were originally 4 individual (expensive) vent elbows and 4 long vent tubes with clear instructions to route them '...well up under the seat for quiet air..." to pressurize the float bowls above the moving air around them. This will push more fuel up in the jets by pressurizing the float bowls more. On the later bikes the top end was sacrificed for less expense and the 2-hose (and later, 1-hose) vent systems were developed. After a while, even those hoses got shorter, mostly because in the USA, Honda's biggest CB750 market, the 55 MPH speed limit caused no noticeable issues for the shorter hoses.
Roadracers knew this, and went to a lot of trouble to solve the float bowl pressure problems at speeds over 100 MPH. The full-fairing of the Vetter Phantom and the [later] Vetter Windjammer with Lowers re-proved the concept on the street, too. On roadracers they made small containers (some with beer cans!) with one or two large holes in locations where only still air could reach them (like way up under the seat) and the vent hoses were fed from there. On the street, I drilled 2 holes in the lower half of the [stock] airbox, as shown in my book, away from the inlet vent(s) and poked the vent hoses in them. At (illegal for 1980s) speeds, this much improved the throttle response, especially in the upper gears. More recently, after my 2013 rebuild that has generated much more hi-end torque, I have found this really helps when I use very tall gears, like 19T countersprocket and 120/90x18 rear tire combo, pushing my Vetter. While this makes 5th gear feel like overdrive that should not be used below 62 MPH, it really improves the freeway passing time in the upper gears.
