Here's a pic of my Winkel Mfg.  3 pt. mover-    The plate that the ball is attached to, is  like 5/8ths.. but it is  cantilevered from  the vertical tube.       

I think you'd be wise to assume your top plate is there just to keep dirt and water out of that tube,  and not rely on it, or it's welds  for any ball mounting purposes.  I'd sure do some 'beefing up'.   

Can you adjust the length of your top link so the old hitch is more vertical? or is the adjuster screwed in all the way now?  Honestly,  it's not too bad, as long as you mind uneven ground.
  Your new hitch is very similar to mine, with the additional pin holes.   It should be a snap to get it leveled.  

I've used the daylights out of my hitch over the years,   and I can say the most important part is to get those sway blocks set to limit side to side slop as much as possible.

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