c/c plates the same for 79-89 as 90-93???

I'm not sure why they would be differant. I just put 90 gt struts on my 79 car. No problem, except the bolt holes were a little tight, or the studs could have been bent a little in the plate on the top mount. At any rate it worked great. My car has slotted strut top mounting holes from factory. I am not sure if the newer ones did. If I remember correctly, my 87 gt did not. I am able to achieve alignment without a kit and my car is lowered about 1 1/2".
With that said, I am a straight liner, not concerned so much about cornering ability. If you are also running skinnines up front like me and are in it for the drag race aspect of it, and not the road race aspect, you may be able to achieve a good factory type alignment setting without needing to lay out the jack for the plates. Another thing to keep in mind is I have seen some companies offer "camber plates" that do not allow for caster adjustment. I almost bought a set until I realized what they were. They were much cheaper.

Im guessing they are different because of the bigger fenders on the 90 up cars. Just a shot in the dark though