As a guy with 17 years on a TKO-600 and multiple installs, I agree with Brent, no reason to go TKO.
The TKX addressed the floor shape shortfalls, and although a customer installed one, and a friend also did a modified housing TKO, the lack of any complaining on the TKX, not overt praise, lets me only deduce that it fits much better. I will ask him specifically though
In terms of the 3rd gear shifting, I do absolutely think a shifter helps. I arguably in 17 years hit 3rd gear more than most, and I have found, it's more due to the lack of spring that allows an old Toploader guy to go "over and up" to 3rd, only to end up in the dead zone in between 3/5 gates. The aftermarket shifters have more of a 5th gear lockout which has to help. That being said, I don't miss 3rd anymore as I know where it is after all this time. Sort of proving the concept, not to mention, if it was exclusively a synchro issue, the other gears wouldn't work well either
I'd also say that an indexed bell is MUCH more important with the tight bearings of a TKO/TKX input shaft, and if running a diaphragm clutch, having the proper clutch lever throw to not overcenter. Each of these can contribute to shifting issues, although not everyone runs a diaphragm, still a valid issue if you do
Last, I cannot prove it helped, but I used Mopar ATF-4 in mine 17 years ago, because I had a ton of it left over. I will do it again if/when I change it, but no idea if that changed synchro engagement, but it is something different I did in mine.
I am in no way saying that better syncro design wasn't needed or didn't help, but once the loose nut behind the wheel stopped trying to shift it like a Toploader, I became very repeatable in finding 3rd gear at 6200 RPM or so
