Here are the pics of my tractor. The spacer under the seat is approx 1".

Unless there is some hidden reason that I cant see, there should be no need to change clutch levers or brake pedals or shifter. They are different on the 7J but on both machines they are forward of the seat (no interference) and changing seats should make no difference. If the winch came off of your 7U then it should go back on with no difference from changing the seat. Since you dont need to change or remove the fenders from your 7U, all of the winch controls should stay the same. You could do the change in stages by picking up a fender tank and mounting and plumbing it and when you find a seat frame, you could remove the seat/tank and install the regular seat. The regular seat mounts a few inches narrower than the tank seat but it should be simple. I personally would mount it a bit higher for the extra leg room, but either way, it should be uncomplicated.
Thank you for the photos of the winch mounts. Those add on upper brackets are easier than the ones that came off of my machine but serve the same function. Mine bolt on to the side of the case, under the fenders, so I will have to pull the fenders and use longer bolts but they are drilled to match the fender bolt pattern. I suspect that all early winch mounting was a dealer, rather than factory job, so differences are not surprising. I have never looked at a newer D4 to see if the rear of the case is different so that the add on brackets may not be needed. I think this will be a much less complicated job than you originally thought. Good luck and please keep us posted. Grant.

Yes I think I could just swap the seat tank out and mount a fender tank but they did use different controls and I would think it wasnt for no reason. Even back then they did have a mind for operator ergonomics. Also when I look at the Hyster D4N manual which is available to read online, I find that there are different controls for the two configurations. I cant help but think that wasnt for no reason either. Have a look at this D4N on a fender tank that has a box on top which is not the same as mine.

Poking around the web, I found that there were 4 different Hyster towing winches for D4s. I had read here on ACMOC that the winch controls were different on some of the models. I have not seen or heard of the operator controls being different because of a winch on the tractor, but that may be. Looking at my U series compared to the older J and G series D4s, I cant see any way to change the operator controls without really drastically changing the ergonomics of the tractor and making it difficult to use. The whole top cover of the transmission and rear end are different on the U model, but the position of the control grips relative to the seat are pretty much the same. If you can find some cockpit photos of a J series and compare them to your, I think you will see what I am talking about. The steering clutch levers are very different and I dont think that the J series levers would work at all. Just my 2 cents worth. Grant.