What weight is your car ?
I think you have weight, and lack of torque on your side.
What shafts are you currently using, what spline ?? Where or how did they break ?
Moser can make anything custom, as can Strange or Mark Williams.
Seeing as I have twisted both 28, and 31 spline Mosers...Im not that impressed with them, but they are cheap at around £250 a pair.
That said, my car is close to 1600kgs, and has a little more torque being applied.
Im now using a pair of Mark Williams shafts, and they are soooooo much nicer than the Mosers. And over twice the price.
I havent removed them yet to inspect, But I think they will hold ok.
I bought mine direct from Mark Williams.
But Andy Robinson Race cars can supply, or Hauser probably.
There are various sources in the UK who can build you a complete custom axle from scratch ( based on 9" ) or opt for an 8.8" like I have, which is fully DIY.