The difference between 5/8" and 16mm is almost inconsequential.
5/8 is 0.625". 16mm is 0.6299". 0.0049", or just a hair under 5 thousandths. It's not a big deal.
Literally splitting hairs, here.
I've no idea what size/spline this is, but from a 1997 Toyota Camry. Looks like a VW rubber steering coupler AND a u-joint!
This looks like what we need, to adapt the VW rubber coupler to the rack, the rack is at a slight angle to the steering shaft.