Smiffy's bits...
Where did you find stainless with the correct tensile strength?
Exactly how I replaced mine last year but used new stainless nuts and bolts and also replaced all bushes with polybushes.If you use 4 axle stands with two pieces of timber it is easy. Jack the car as high as it will go at the back by jacking on the subframe side members. Put two axle stands each side under the rear of the floor with a piece of timber about 12" x 3" onto each pair of stands. The rear edge of the wood should be in line with the end of the floor.
Lower the car onto the wood, undo everything including the exhaust, hydraulics, handbrake, dampers, and anything else attaching the subframe to the car, then remove all the subframe trunnion bolts and lower it from the bodyshell.
Not the correct grade. Only warning you as modifications like that both void insurance and are life threatening.