Hi Eddie - sorry I've not been about for a while (you can guess what I've been up to
), but I did have a thought about your suspension problem - maybe its not the spring rates, but the anti-squat geometry.
I seem to remember you saying you had a Capri rear axle - is that a Salisbury or an Atlas? Anyway, its a live axle, presumably 4-linked with a Panhard rod or similar for lateral location. Are the trailing links parallel to the vehicle centreline? Is there an upper and lower link each side?
I'm asking because the dragster chassis (and a few racing setups too) use a trailing link mounting plate with a series of vertical holes, so you can set the anti-squat geometry. As I don't have enough power to develop any squat, I know diddly about it, other than you project a line forward from each of the trailing arms and they should intersect at the vertical CofG point for perfect anti-squat. The idea then is you can use the softest springs you like, so that the tyres stay in contact with the road instead of skipping about all over the place.
I had the same problem with my springs - 8" was too short, 9" was too long, so I had to preload the spring on the front shocks, and at the back I got perfect ride height with 10" springs but the shocks extend to 14" at full droop so I had to use helper springs the keep the spring collars in check!
Sounds like you are making good progress, all told. Well done!