For alignment purposes, camber on the rear of the TR6 is a function of the static installed spring height and the relative angle of the pivot line of the inner and outer trailing arm pivots/brackets. Spring rate comes into play when you are talking about changes in camber and toe as the suspension is loaded or unloaded and goes through its range of motion. Buckeye Triumphs (IIRC) has a nice chart of the relative static camber angles using various combinations of the three different stock brackets in notches up and notches down orientations. You check your camber and note which brackets are installed and the notch orientations, then consult the chart for the camber change needed and see what bracket combination is recommended to get that change, then give that combination a whirl and measure camber again, repeating as necessary. For a while this chart was also included in Moss catalogs but then it was dropped for some reason (perhaps copyright considerations). It can be done and it's not really that hard, but it can turn into a bigger effort than planned on since it is kind of a trial and error process (have first hand data on that). The rear toe is adjusted by means of the shims mounted between the brackets and the frame.
Terry_M, you state that you "have them adjusted fully out." By adjusted fully out, do you mean that you have all four adjusted to the same position either fully lower or higher in the brackets?
There was a TSB for TR6s put out by BL on rear camber which involved using a spacer on the underside of the rear springs to bring rear camber to the specified -1° ± 1/2°. The spacer was supplied under one of the HAC series part numbers but I don't specifically recall the exact number other than the first digit (HAC 1??). Unfortunately it was kind of a one shot deal and since it called for placing the spacers on both sides it did not really address the issue of camber differences between the sides. I know of cases where a spacer was fitted to one side only, the spacer fitted to one side was machined to be thinner or shims placed under the spacer to even things up from a camber standpoint and usually, but not always, to address the wheel center to fender lip distance.
Also note that while your car came later in production, there was a recall campaign on 71 down TR6s on front headlamp center height that involved changing out front spring pads to thicker ones. MG had a similar issue on 72 down MGBs, but to address theirs, it involved a spring swap out to get the headlamp centers to the federally mandated minimum height.