U-joint for steering box.

I ordered a tilt column for my 71 dart. It is a typical GM column. I also ordered 2 u-joints to connect the dd shaft to the gear box. I ordered 3/4 x 5/8-36 spline. The dd side fit but the 5/8-36 is .630 too small to fit the spline on the gear box. I measured the spline on the gearbox and it is .617, so the 5/8-36 should be correct. I contacted the seller and they said that it should have fit. I sent them back. Now I am unsure of what to buy. Side note-gearbox has master spline but the u-joint had a flat spot approximately 1/4 in wide at the top. So where to know? TIA.