The Sierra donor brake back plates do not fit flush with the Tiger provided rear suspension uprights. The extremeties of the flat angled plate foul with the curved bulges on the back face of the brake back plates.
A small amount of metal has to be removed from two places on the rear uprights. The areas to be removed can be marked out with a permanent marker pen with the back plate pressed against the upright. Then an angle grinder with a thick metal cutting disc can be used to remove the offending portion of metal. Removing small amounts at a time and repeatedly trial fitting ensured that only the minimum of metal was cut away. Thereby ensuring minimal reduction in the structural integrity of the upright.
Here is a view of one of the uprights after surgery. The bear metal surfaces will be painted with smooth black hammerite prior to fitting of the brake back plate.