The 750's tank (in the K0-K6 series) changed 4 (or maybe 5) times.
The earliest ones, K0 series, were wider and lower at their top. As Honda started working on reducing the "tallest bike in the world" problem to improve sales (by widening the potential buyer market) they slowly made the tank narrower and taller until the K5 model. This changed big-time with the narrow F0 tank and the very teardrop-shaped K7/8 tanks.
The 'steps' as I noticed them were:
-The [post-wrinkle] K0 tanks were the widest, almost fully blocking the view of the engine from the riding position. All I could see of the upper engine was the outer edges of the #1/#4 intake tappet caps, and the sides of the cam cover.
-When I got my K1 I noticed right away that I could see the outer portion of the hexes on the valve caps (where the wrench fits) and the top of the gas cap was level with the top of the handlebar, in between the handlebar clamps. This was a surprise to me at the time.
-When I got my K2, I purposefully looked down and can (still) see almost the whole hex of the outer valve caps, and the gas cap at first felt like it was protruding in my hand when I went to fill it up. The cap's neck wasn't higher, but the tank obviously was: the distance between the bottom of the tank and the top of the engine was, though, less: I could reach in over the valve cover on the K1 and touch the cam breather hose, but not on my K2. So, the tank is taller, narrower, and set slightly lower than on the K1.
-When the K3 came out, it started with the K2 tank. By its end, though, the bike was receiving the tank of the [upcoming] K4, except the petcock was still a dual-output tube on the right side. I have one of these latter tanks in my shed (it's rusty inside, needs the POR-15 treatment).
-The early K4 had the right-side petcock, but about 1/3 thru the run it had switched to the opposite side with 2 outlets and a spigot on the tank, to make it miss the choke lever on the carbs. It sits so low that the hose makes a sharp bend immediately out of the petcock to make it thru the carb bracket's hole and hit a tee there for the 1-2 carb feed drop (The F0 tank has this, too). This one tank setup ended with 1-hose oultet on it, but the petcock can be drilled for another, or the 2-holer subbed in its place. This will resolve the fuel-feed struggles above 65 MPH that are due to the 1-hose arrangement. (See also the comments below about vent hoses.)
The K5 (and later K6 'revival' version of the 'K') had single-outlet petcocks on a spigot on the left side of the tank, and when K4 tanks are used on these bikes the choke lever can interfere with the petcock if the petcock lever is the early (longer) handle. The K5/6 tank sits slightly further forward and is as tall as the K4 tank, and as narrow. It holds 0.15 gallon less fuel, too. On this model, the steering head's stops are moved narrower a little bit to prevent contact between the fork's headlight mounting ears and the tank in full-angle turns of the handlebars: if the earlier tank(s) get installed on this frame the headlight ears can actually dent the front of the tank, especially on the right side.
Part of the "raising of the tank" was likely due in part to Honda's making the tanks narrower over time to accommodate a narrower seat, sidecover, and tank interface for the rider's thighs, all to make the bike feel less tall. The change of the shocks' length from 12.8" on the K0 to 13.1" on the K1 happened when the side covers were smoothed off into a roll (instead of a box corner in your thighs), so the feeling of the bike being very tall did not change. I really noticed it, though, when I bought the K2 (and the K2 also has a unique seat that one year, with scalloped sides where your thighs go when standing, and the seat's top bead flows down there and back up to the passenger's position - which, BTW, lets your jeans scrub the paint off the tops of the side covers over time...) because, compared to my K1, the whole bike felt like it was shorter. It wasn't...
Another part of the tank changes was (quietly published as) the fuel-feed trouble the K0 had at high engine speeds in (then high Interstate speed limits) touring. It was relatively common for the K0 to singe or outright burn exhaust valves #1/2 if the bike was run at, say, 90+ MPH on Reserve for long periods of time, because the fuel levels in the carbs fall, especially in carbs #1/2. This is still true, BTW, with the K2 tank (mine does exactly this) and was resolved by the K3 mid-year version, and the K4. The K5 suffered it all over again, but the speed limit being 55 MPH then hid the trouble well. The F0/1 tanks were much taller and put almost 0.1 PSI more pressure on the fuel (above Reserve), which solved it, too, on those single-line feeds. Interestingly, the F1's petcock outlet faced inward instead of downward (like the K7/8), improving the fuel feed by having the fuel line run across in front of the carbs instead of turning to the #1. This isn't often seen in much-reworked bikes today, though, probably thanks to the Internet...
In the end: whatever tank you get, make dead sure to install long vent-tube hoses on the carbs' bowl vent elbows (the K0 had 4 of these, this dropped to 2 by the K1 and just 1 by the F2/K7). Route the ends of those hoses either back up and over the airbox to the still air behind it, or drill some small holes into the [lower] front of the airbox and plug those little hoses into the lower (below the air filter's mount baffle) half of the box. This stops the rippling air that rattles over your legs from hitting those vents from alternately pushing the fuel back up the fuel lines and sucking it back down to one side or the other, making subtle fuel level 'bugs' that can drive you NUTS...(roadracers, please take note...).
If you are really after that last little bit of fuel-flow, put vent hoses on all 4 carbs and run them either up under the seat or make them real long and end them behind the battery, above the rear plastic fender.
