Sorry to put another worry in your saga 3Gs I really think you will be OK if the fit is very tight. Not many failures have been reported. I think my upgrade inserts have found one or two of the weakest examples - including mine.
to demonstrate my concerns top picture, above, shows a brand new Centa coupling with a small section of original, unworn, S&S shaft inserted. The picture was taken with back lighting to show the gaps at the bottom of the splines where it does not touch ( minor diameter ) and you can just see a bit of light at the top of the splines (major diameter ) where the fit is better but still not perfect. This design would be OK with a tough alloy steel female spline, similar to the shaft, but the Centa couplings come with a soft steel spline which needs a better fit.
The second picture ( also back lit ) shows how it should be.
