68 Camaro power steering coupler / rag joint

[gmart68ZRS]

I'm replacing the steering coupler on my 68 Camaro. The shaft on the box is about 11/16 dia, (a bit smaller than 3/4) with a flat spot. The new coupler is for a 13/16 shaft with a flat spot. Both couplers have 19 splines from flat spot over to just before the pinch split. New coupler fits way too loose. I did replace the power steering box 10 years ago. It is the quick ratio box 2.5 lock to lock. But now I'm questioning if it is really a Camaro box. I ordered it for a 68 Camaro, and it bolted in. But the shaft size is smaller. Are there two (or more) sizes for the shaft, like 3/4 and 13/16 ? Is there one that is smaller than 3/4 ?