I have a similar problem currently "in the works". Both my cars have a 65 ltr LPG tank in the main boot well. We have lowered the boot floors by about 2" so that the tank sits below the threshold of the battery tray / tool roll ledges each side, giving us a flat boot floor across the car. Observation suggests this won't give any (additional) problems with tail grounding as the floor is still above the central "jacking point" boss.
I too am concerned about rear end weight. Currently we have +35% front springs and +45% rear springs with standard ride height on the English car, the Thai car running a full Ray Weekly uprate setup, which we believe are +25% front and HD rear. (the Ray Weekly HD rears are a progressive variable rate spring so you can't quote a simple +% on the spring rate) The intention is to have the rears about 10% stiffer than they would otherwise have been. Only time will tell if these settings work or not and I fully expect to have to change rears on at least one of the cars!
Given that your rear springs are not otherwise "tired", then there is at least one get out of jail tweak. Rover specified a rear ride height based on a body to ground dimension. To achieve this against naturally varying spring quality there is a provision to fit spacer washers inside the spring pockets. Solution - fit some extra spacers!! (any breakers can provide!)
There are a couple of other observations on this problem. First, raising the rear spring rate has the desirable side effect of improving the rear roll stiffness (ie the back end won't roll as much in corners). If you haven't already got oversteer in corners this will give it to you - great fun!! (remember the graphic slides with P6's on The Sweeney - they would all have had the uprated rears) The second effect is that you have introduced weight a long way out from the centre of gravity of the car. This will increase the polar moment of inertia (bad!) - in simple terms the car will be less inclined to change direction suddenly and once moving (ie sliding) will be less inclined to stop. Most people are only likely to notice this latter effect on a race track!
Hope that helps
Chris
