I have yet to put any bushings on my old flail mower. Put em on pretty much all my other impliments. Several have that knob type for the bolt on pins. I ended up going to the steel place & got some pipe & tube that would get me from Cat1 to Cat3 diameter. Have 20' of that now to use for future implements unless I forget & build something out of it.
Those knobbed ones are nice for some sloppy implements that I don't trust not to shift around (my subsoiler comes to mind). Pipe is great for the clevis mount style though.
I just welded a 2" extension onto the back of my 4th QH top hook so it sticks out further. Other QHes got sold with the old machine or were for friends. On many of my implements the implement would hit the QH frame before the top hook could grab the top pin properly. Welding 1-2" onto the back & re-drilling the holes meant the hook sticks out further & grabs without interference issues. Can't tighten up the toplink as short because of it, but that was never really an issue for me.
Never had any real geometry issues, other than my PHD. I didn't measure enough when I cut it up & welded it back together to fit my QH (That only works with a hydraulic toplink by the way).
My ancient flail mower needed a new PTO shaft. It was 2" to short to touch, much less get the proper 1/3 length overlap, so I'd have had to get a new shaft regardless. Have yet to mount my 3pt trencher I recently picked up. Each half shaft is 6-8" so I might need a new longer one (might even without the QH).
As others have noted... modify the impliment so it will fit a QH. Your life will be so much easier in the long run. With TnT & a QH on my old machine I could hitch up to anything that didn't have a PTO shaft or hydraulics in 30 seconds without getting out of the seat no matter how crooked the impliment was sitting. It's easy to do if you can weld at all. I never used those adapters, just extra steps to hitch up.
I replaced the top hook bolts with pins so I could move em easier. Although really the only thing I moved it for was my LandPride rotary cutter. It had an flex link on it that would pop out if I didn't move the hook down. In retrospect i could have probably just removed the Cat3 bushing & pined it onto the QH. But it was 15 seconds to move the hook down & flip the link in after I hitched up the lower pins. I had to get off for the PTO shaft anyway.
I'm so annoyed that my TnT cylinders for my new
L4060 aren't in. Have to get off & crank on links to hitch up. I can see the value of the extendable lower links now, but the QH kind of negates their usability. A QH & TnT is much easier than the extendable lower links though as there is usually no need to get off.