Can not find specs for front and rear coil springs questions
1. are front and rear interchangeable, are coils same Coupe vs Conv 4.0
2. coil height unladen
3. what tool to use to determine if spring has failed due to age/miles
4, shop diagram for measuring ride height doesn't say what, if any, difference (lower) is acceptable. My conv shows @1.5" lower than spec both front and rear. No issues yet for tire rub, suspension noise or road scraping, but speed bumps are getting jarring.

 

There are many threads on here about that topic.Front and rear are not interchangable. Ride height is measured with a full tank of gas.Springs being different for coupes and convertibles im not sure of ]. Dont know what tool might test springs. I have found nothing on how low is acceptable. There are packing rings in the shock/spring assembly that can be added or removed to provide some adjustment. Going that way may involve removing assembly more than once to add or subtract from the ride height. Generally go with what was there and you can get new ones through several suppliers if they are broken and need to be replaced. I am doing the rear springs and shocks (dampers) on my 93 4.0L convertible.. You cant get the original springs anymore, I do believe. I ordered 4 for the rear from Moss motors. Just shocks may or may not bring your rear height back to spec. I am doing my springs because she sat for many years with bad shocks and I feel the springs arent quite up to what they should be. You may or may not get precise factory ride height even replacing both. I did replace just my front shocks and am within 1/4 to 3/8s of spec when I finished. I have a bad head gasket, blow through between 4 and 5 cyl. and will do the tensioner blades at that time.I am also going to get a rebuilt steering rack. will also be doing some other age related work. IRS was done last year except for shocks and springs. I highly recommend getting the Shop Manual as well as the parts book. Both are available on CD or download from Ebay as well as others. Might be able to copy mine to a thumb drive for you. Good luck and keep us up on what you are doing.