The problem with coil springs is most are generic spun wire, and tolerances are all over the board. 'Accepted' tolerances are in the 8-12% range, which means for example, on a 500 lbs spring, can vary from 40 lbs - 60 lbs. Imagine if you have 2 springs at the end of the tolerances? 440 - 560 from side to side, and you get the dreaded 'lean'.
Coil overs. I asked that question in the 80s, and it was explained to me in this way:
The longer the piece of 'wire', the more granular the rate changes as it bends.
Imagine this. You have 2 identical thickness metal rods, one is 18" long, and the other 36". If you grab either, and you slowly apply pressure to bend them, the longer one begins bending 1st, right?
Traditional coil springs are 5" or 5.5" diameter. Most are in the 16-20" free height range. Traditions coil over springs are 2.5 diameter, and in the 10 - 14" range. If you take one of each, and straighten them into a rod, which one is longer?
Packaging coil overs and strut/spring assemblies is much easier and cheaper to use and produce. In a performance app, generally, above a certain weight, all the OEs use strut/spring combos, because the coil spring on a strut is much longer 'rod'. An exception is on lighter cars, and in canted/cam type suspensions.
GW is now crossing over to coil over setups, and it looks like they are basing it off the QA1 design(s), so if you want to go that route, the QA1 stuff is the best. The 'hybrid' setups with tapered springs, are basically a strut/spring setup. The 'Pro' stuff is cutting edge, excellent, but not cheap. That also is a factor for me, I already have an investment if the traditional coil spring suspension (control arms, bearings/bushings, etc). Each shock is around $2k ea, and you basically need to run their' control arms too. Front and rear kits are in the 7500 - $8400 range.
https://qa1.net/GM/Car/GTO/1968-1972/Handling-Kits/
Its still somewhat hybrid, the springs are still tapered, but it's built for heavier cars.
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