Depends on what you want to do. You can go full tilt and get somethin glike th eMagna 620's that Mustangsplus offers (always kind of rough, but very good for road racing). Or, you can go with the Eibach progressive rate springs, that on grocery store trips are fairly smooth, but get better when you need them (roadracing).
I've got the Magna 620's and like them. However, as I gracefully age, I suspect I may want to replace them in 100 years or so, with the Eibachs. hehehe
Steve Leslie, 65 coupe in restoration. 302, toploader, A/C, disc brakes, bench seat
I got the GT rate springs for my '65 from Mustangs Plus. I've had a bit of a problem with them. They really aren't tall enough. They are 1" lowering springs, and I also lowered my control arms 1" (which makes about a 1/2" drop in ride height). I'm using the taller spring insulators and still have to use a couple of coil spacers to keep the wheels from rubbing the fenders during hard turning/braking manuvers. I think the GT's would be fine if you could find them in regular height (without the 1" drop) OR if you aren't doing the Shelby control arm relocation. I'd guess they are about 300 lbs/inch. Don't get confused my these vendors that list 640 springs. That's the wire diameter...not the spring rating. Good luck!
If I recall correctly, the GT springs were rated at 480. I've used 620s before and they are too stiff for my taste. Others love them, however. I bought some variable rate coils that are 320 - 480 (I think). I'll put them in when I replace the front suspension this summer. I installed GT rated units in the rear last year.
I went with the 620 lowering springs, but I also have all the weight of a Cleveland up there, and I'm still young and stupid. I've heard lots complain that the 620's are rock hard on lighter cars, but it all depends on what your personal tastes are.
'72 Mach 1
'90 Talon TSi AWD
"Thats not a leak, my car's marking its territory!"
"If you've done it, it ain't braggin'." -Roy Rogers