Steering input shaft size

[intocarss]

I know there's 13/16" & 3/4" input shaft sizes..Here's my deal

The box that's out of my 68 Camaro on the bottom of the picture is a 3/4" shaft. And the one on top has a smaller (maybe 5/8" or metric??) input shaft. The rag joint used in my car fits snug on the 3/4" shaft but fits loosely on the IROC shaft.

The box on the top (in pic) is a mid 80's HX IROC box. Other box is mid - early 70's box. the converstion rag joint I have only comes in 3/4". I've done a search and no mention of the smaller shaft or a 5/8"/metric rag joint ..

If I tighten the coupler bolt ALOT, it will crush the coupler so it grabs the shaft but that does not seem the correct way to do it. The OEM coupler from the 80's Camaro will NOT fit onto the 3/4" shaft (did that just for refrence) .. What am I missing here??