I found the best way to compare different springs and not have to take the tire diameter come into play, is to measure from the center of the wheel to the bottom of the fender lip. If spring X creates Y inches from the center to the lip it will do it whether one has 20 inch diameter tires or 29" diameter tires.

I've had my front springs out about ten times now. Between trying to find the right height or the correct geometry, the A arms and springs were frequent R&R items.
It's almost as if I should have purchased twenty sets of coils, then R&Red them until I got the ones that gave me the stance I wanted. There were a gazillion different coils made for our cars, most manufacturers only supply a few. I installed the Moog replacement for a 68 non ac, manual brakes, power steering car and they were about two inches too high. Go figure.I installed a set for a car with every option available and it sat too low??? Some guys buy a set and they are perfect, every car is different and I'm starting to think two sets of springs with identical part numbers from the same manufacturer will be different. The correct height of a stock car can be found in the AMA specs section on this site. It is measured so many inches from the front and rear wheel centers and so many inches from the ground to the rockers. You have to remember the stock FGF where a little high in the front so ordering a stock height spring may be a bit tall for your taste.

One has to look at the condition of the chassis as well as the springs. Worn, bent A arms, sub frames and cross menbers will also affect the ride.

As far as the coils, it would be handy if you could measure the spring rate of the springs you now have on the car. Take them out and add force/weight until they compress one inch. You would then now what the spring rate is and if it's too soft or stiff you could get a set plus or minus what ever it would take to get the stiffness you want. That wont help with the ride height though.

You could call Eaton Spring, they have a good variety and can make springs for a dropped or raised look.

One thing to consider when lowering the body is whether the tires are going top rub on the fenders.

I R&Red mine a bunch of times, cut some, put spacers under some, replaced then modified the A arms and continued to get tire rub unless I had the car sitting too high for my liking. I finally bought a set of adjustable coil overs with a higher rate spring and now it sits where I want with no rub. I should have done that from the start but didn't think I should spend all that $$$. Would have been cheaper if I spent all that $$$ from the get-go. My car has been bashed up at least once so things are not quite as they were from factory.

The rears were replaced some time ago, mono to multi. They were so worn out air shocks were needed to keep the body off the tires. The mono spring perches were not replaced when the conversion was done so I had to weld some repairs there. The only place near Sidney I could find rear leafs for mine could not get Firebird springs with the rate and load I wanted but said they had good luck putting Nova springs on Firebirds and Camaros. I bought a set, plus a couple shackle/bushing kits, and installed them last winter. Bushings didn't fit so that was another $100 but I got it all installed. It sits a little high at the back but I like the 70's Muscle car look with the back raised a bit and larger tires on the rear than the front. I'm happy with what I have now but it took about eight years and a lot of skinned knuckles to get it.

The shocks are not what keeps the car at the correct height, it's the springs, the shocks keep it from bouncing, keeping the tires on the pavement. If yours is dragging it's belly the springs are sagging. Replace the shocks as well.

The springs did out last some of the early Firebirds, the ones that were wrapped around power poles. Every thing wears out eventually.