A stock 1970 510 speedo cog pinion does fit on a 90s era KA trans.. What they did was keep the same design for the digital sending unit and rotate the offset just about 180 degrees. I have gotten the stock speedo to work mechanically by simply rotating the speed sending unit until it fits in and then cutting a new retaining slot where one needs to be to correspond to the stock bolt hole in the transmission. In the five ten you need a longer speedo cable because of the longer ka trans.
if you dont care about stock speedos maybe you could just do that in reverse? Take the later model digital speed sending unit and adapt it to a speedo that is adjustable? I don't know for sure, just an idea I have had floating around.
Its not too hard to pop the needle off and mount a stock faceplate to newer speedo internals.
For myself, since I have never gotten different gears to be accurate throughout the range of my engine,trans,diff...( I have a dogleg in my 521.)
I shoot for my speedo to be accurate at around 60, rather the freeway speed near me. Simple so I know when I am speeding for sure, when I pass that cop pointing his gun shaped speed sensing device at me.. Otherwise I just drive like I normally drive and ignore the speedo. I care more about the tach personally. In America we have digital speed signs; In the past I find one close to my house, and test out the speedometer by driving past it at certain speeds. It is also still legal (as for helping a driver) for someone in America to ask a nice policeman to drive the speed limit and have you follow them so you can check your speedometers accuracy.. might be able to get out of a ticket by politely asking for that as well.
I mean you just swapped transmissions and put bigger tires on, even though it felt shaky at the "highway" speeds, you were just trying to get home so you could adjust things. All you "have" is your guages and they may have been affected by the mods..