You have it right: the #120 mainjets are too rich for today's fuels. They were marginally too rich for the 1970s fuels, too, but Honda was afraid of riders buying Regular gas instead of Premium when the 750 first appeared, so their Warranty guys overrode the Engineers, 'twas the scuttlebutt at the time. By the end of the sandcast, though, they were already showing up (most, not all) with #115 mains, and by the end of the K0 they were sometimes #110, too, with no explanations. The needles in the K0 bikes were all in the middle notch, even the sandcasts, which sometimes had different (shorter) slides and needles from most!
All of the replacement floats I've seen from Honda since the 1990s are the plastic ones. They will work, but act slightly differently (i.e., slower to settle closed) from the brass ones. Specifically, the brass ones were always set staggered by -1.0mm on one side (the "sidestand" side of the bike) to 25mm for that float, with 26mm for the opposite one, and this practice remained when the K1 carbs started getting plastic floats near their end of production. On those floats, the tab that pushes closed the float valve is also at an angle, mimicking the offset floats' "tilt". This type of float-height setup is/was found in the carbs that have the tiny sliver of a raised casting inside the gasket area, in between the edges of the gasket surface and the floats. This is also the reference fixture for setting the floats in that carb. Note these are #657A carbs with the crank-style throttle link, and most of the cable-style carbs, that contain this little 'edge'.
At the end of the 657A carbs came their "B" revision, along with the HM341 pipes. From the Old Factory these had #115 mainjets at the very beginning (like 9/71 thru about 11/71 builds), with #110 from the New Factory: the cams were also slightly different in the engines between these factories, hence the jetting change (and resulting confusion in the intervening 50 years!). All the way thru the K3 model, the mainjet was #5 richer in the Old Factory bikes, with #115 or (after about 12/72 builds) #110, with #110 or #105 respectively. All the K4 and later ones were #105 in the roundtop carbs of all numbers (like 086a, 087a, 7A, etc.). All of these carbs in HM341-equipped bikes raised the needles 1 notch to slot #4 to improve on a slight flat spot at about 1800-2400 RPM that is caused by the increased back pressure disturbing the flow thru the carbs until full spark advance is reached: this is why, 50 years later, the too-soon advance of the spark with heat-annealed softer springs in those advancers causes low-speed stumbling and dark sparkplugs now. Tightening up the advancer springs makes the carbs behave better (honest!), or else you can drop the needles back to the center notch, which sort of works (with a LOT more effort).
All of the K4/5/6 bikes had the same jetting from the factory, #105 mainjets and #40 pilots with #4 notch on the needles, and plastic floats. The floats were both set at the same 26mm height and the gasket surface (sans gasket) was used for the 0 height reference, in between the notches you can see in the main body: these carbs were made in better molds, too. The float tangs in the later bikes were all set parallel to the plane of the float bracket, no more tricky angled tab there. Note that things changed with the F0/1 bikes with the roundtops, but that's another topic.
So, when the internet exspurts tell you all they know is absolutely true about the 750's carbs' jetting without having seen the entire parade of these bikes, you can now be smarter.
In all cases today, the gasoline changes we have require less octane to burn at the designed-in fuel-burn rate. So, don't run Premium gas in city riding, or you will suffer fouled plugs, or at least shorter life with them.
