If you are going to change the spring pads you would be advised to change the springs at the same time. The labour cost to change the pads is the same as it is to change the springs. Pick up a set of front springs from a dismantler near you. Go and have a look at a wrecked 420 and get some springs from it if it looks okay. In your last posts you were talking about front ride height. From what you have said in your earlier posts I don't believe that the front is the problem, I think it is probably the rear that is causing the harsh ride. Certainly the rear springs could be sagging, combined with the harder valved shocks and the standard front this could be causing all your problems. I would message the clearance of the front and rear wheels on both sides and post here, hopefully someone with a 420 can respond and confirm which end is not at the right height. I would also guess that the cd should have this info so someone could look it up for you.