Karl,
The brackets have to be welded to the torsion housing, at a location that requires clearancing the top and bottom sections of the frameforks, where they meet at the tube. This clearancing has to be very neat so the brackets can be fully welded to give the necessary strength.
The importance of correct alignment goes without saying.
If your skills are up to an IRS conversion, fabricating a pair of fixtures will be easy. These fixtures should pick-up the position of the trailing arm bolts/mounts on a genuine IRS 'pan. Rigidly connect these to the swing-axle springplate covers bolted to the IRS torsion housing. The fixtures will then unbolt and bolt up to your SA 'pan and you'll have a bracket mounting point to work from.
I appreciate the inconveniance this may cause, as you'd need a donor 'pan. If so, consider a BJ framehead/(Chassis number?
) swap for a better result (IMO)
Matt