I spoke to Farooq. From what I understand, the way it is DONE is that they cut the shaft of the off spline in a slight V (probably at both ends), weld the tips of the two Vs together and then fill up the open part of the X with weld. Then they machine the final shaft for a finish. Heat treatment I never bothered to ask about
To date there are no reports of breakage of the welds whenever done properly by one of the known lathe guys. Only the lathe guys do this BTW. There are some reported breakages in vehicles of unknown conversion origin, nevertheless not a common issue for any of the Jeep and FJ conversions. The conversion records at CSR stretch back to the mid eighties and most major issues and new 'jugars' are shared between the main conversion specialists as well as the general market.
I am sure the process could be improved on several counts starting from better quality welding wire. The rest of the metallurgy I don't have any views on except that I am not motivated to really fix anything until it breaks. Test cases open to public scrutiny are Jadoon's Nissan with 1FZ and my FJ with a 1UZ. Both 200+bhp and more importantly 250+ft.lbs weighing 1800+kg having withstood quite the ragra in recent times.