Henry Rifle is right on about the Slovers. They are a one of the best vendors in our hobby. I hope they continue to prosper because they deserve it for all they do for our sport.

Your coupe's weight is right on with what I would suspect. And congrats for having such a balanced distribution between the front and rear. This should prove to be a blast to drive once you solve the spring issue.

I would recommend trying to determine which brand of springs you have on the front. Most coil-over manufacturers will swap out springs with you if yours are not worn too much. And at the very least they should only charge you a small exchange fee, so definitely ask them if they will. You might be surprised. I would even ask them to send you several spring "flavors" so you can do this in one day. Even if you have to pay a deposit, it would save you money in the long run by shipping only once.

As for the rears, again see if you can locate a small painted dot or stripe on the last one or two coils. This would be much easier than taking them in to be tested for a pound rating.

Your A-arms should be level on a mustang. If they are not, someone could have installed a shorter spring on your shock or they improperly adjusted the spanner ring to lower the car to the desired "cool" stance. Either of which would explain your harsh ride. I had a chassis built by the guy who used to build Alloway's frames and he got away with that trick...until the shock bound up and snapped the shaft in half. Then I found out what he had did...I wasn't happy considering I paid top (and I mean TOP) dollar for his work. Anyway...check for that.

How much travel does the shock have before it bottoms out? Can you post a picture of the front shocks mounted on the car? If you can it would be a great help in assisting you.

If not, simply remove the shock and :
1. Measure the total length of the shock bolts eye-to-eye.
2. Disassemble the shock and compress to measure the compressed length.
3. Subract #2 from #1 and this is your total stroke length. Your compression and rebound should be about the same (give or take a very small amount)...so if you divide the stroke length in half and subtract this amount from the overall length, then you will know how long the overall bolt eye-to-bolt eye distance should be at "loaded" rest position.

Obtaining the correct ride height of your coil-over is critical. If you say your car jars your back...I would imagine it is bottoming out before it can rebound and support the weight smoothly. It certainly wouldn't be the first time a builder sacrificed the ride to make the car "sit" just right.

Now lets move on to the spring rate. Matching your spring's ability to support your car's weight is heavily dependant upon your stroke length. The shorter the stroke, the quicker the spring must react and rebound the opposing force. This results in a harsher ride than a longer shock...makes sense right?

So try to determine how much rebound room you are allowing your coil-overs perform with. You might be asking them to do something which is difficult or near impossible. And then swap out the springs in no more than a 10% variance interval until you smooth out the ride. If you end up raising the car to make it ride correctly, then perhaps invest in dropped spindles. You should be able to find middle ground with both looks and driveability.

Oh yeah, you asked about when my Durant Enterprises spring broke...75 mph (at night) and breaking a spring was scary! Fortunately it broke in the center eye so it just sat down on the axle and was held in place by the crossmember and spring bracket. Other than a quick "Oh SH#* and a wounded wallet, everything worked out. I was able to limp home and change my undergarments. But I still love those dropped axles!