Many thanks to John Livesey for this interesting article:

This is a genuine photo of a prop shaft I fitted whilst doing some other work ( I was fitting a new diff as I remember).
I was going to change the UJ's on my original shaft as a matter of course when doing the work, but someone recommended these new items at a reasonable price, so I thought I'd give one a go.
In fairness to the supplier, I broke this doing practice starts the evening before I was due to take part in the Blackpool sprint and the tarmac I was on, a private industrial estate road, was very abrasive/grippy.
I reckon my engine at this time was putting out about 280BHP (it's been modified again since) but interestingly my old prop shaft, the original unit, handled the power fine and is still doing so.
By the way I did win the modified V6 class.
On inspection it turned out the new shaft was made from a thinner wall tube and was a slightly smaller diameter tube.
There's no moral to this tale except you live and learn.