Not sure about all this "strengthening of the rear end" business! As far as I am aware, any rear coilover setup would work the same way as mine, putting the coil spring loads through the strut top mounting via the normal rubber isolator.

My setup runs a fairly soft rear spring as the back of the car is fairly light so the increased loadings are not fantastic. Remember that the standard spring that runs between the wishbone and the chassis rail is considerably stronger (maybe 300lbs/in) due to the leverage effect of its position with respect to the wheel and the inner pickup point, so the loading at that point would be relatively large compared to that at the top of a strut mounted spring, which is direct acting on the wheel.

I ran my Mk4 Escort rally car for several years with the identical setup to Tony's RS1600i rally car, with NO additional welding at all to the rear strut top mountings. There was never any sign of stress or cracks appearing. A clamp on rear strut brace would be a good idea though.

Martin