Wrong starter
I can't be sure if it's the entire starter or the starter drive that's wrong.

But I suspect the drive is wrong

Also it may be the nose cone on the starter is wrong.

It is definitely wrong....
As I mentioned, I have swapped many of these and never and issues

You said the same thing with the original flex plate ????

OK

I Dragged out a factory Delco starter I took of a 6.5 (My own)

Get yours off the rig
Measure the drive teeth diameter (1.264") tooth to tooth
10 tooth count

Lay a straight edge across the mounting flat and measure the distance to the top of a tooth that's as close to parallel as possible.

1-1/4 to 1-5/16" (My nose bushing is a tad loose)
From the edge of the straight edge touching the machined surface where the starter mounts to the tooth tip (Outer diameter)

Minor differences may be seen...

This is gonna tell the story.

I suspect either an improper drive or the nose cone is wrong one or mis-machined