Lift Springs/Kit

Well, that might have been me, I did tell you that I put '84 Thunderbird REAR springs in my old '90 (along with 5.0 front springs) and they lifted the car around an inch or two.

I don't know about the front springs, maybe they aren't taller.

I don't know how high you're looking to go, but JCWhitney sells "ultra high lift" springs for '79-93 Mustangs, front & rear. Says they lift the car 2-3" or something like that, I doubt you would be able to make them work with stock suspension geometry but you could probably cut them a bit.

the TBird springs are indeed stiffer since they weighed considerably more than a like engined Mustang

I looked at a set of 88 Tbird springs here and they are way taller.