Not a suspension expert, actually a real layman when it comes to suspensions, so take this FWIW:
- the shocks likely exert a mild upward force on the body and thus the springs. If the springs are really tired, they may ´give way´as a result and stretch out in the front, allowing the front to lift, when springs in good condition would not stretch. Otherwise, the springs and the weight of the body should not allow the mild upward force of the shocks to influence the ride height
- also, if the rear springs are really worn and sagging, more so than the front ones, then they may be allowing the rear of the car to ´sink´; if the front springs are in better shape, keeping the front where it should be, then it only looks like the front is sitting too high, when in fact it is the rear that is sitting too low
My advice, especially given that you are going to drive the car in rallies, is to get 5 new springs and to go for the progressive rate ones. A group member once let me drive his car with as I recall the John Olson supplied progressive rate springs, and the ride was really nice and certainly no longer had the nose dive on hard braking that is so typical of our cars.
I´m sure more knowledgeable members will chime in soon and correct my amateur advice!