I disagree, as Eaton Detroit Spring never sold OEM parts to GM. You are thinking of Eaton-Dana/Spicer-Federal Mogul Corp. (which includes TRW); which was an OEM vendor.
I have heard, as a Mod on four boards, a lot of complaints about Eaton Detroit Spring's Customer Service (if you order the wrong spring it is yours; they don't want it back: to wrong spring in box, or box was was mislabeled).
I personally bought a set of replacement springs that where wrong, but since I could not provide them with a four corner weight Eaton Detroit Spring said it was my fault and I ordered the wrong spring. I agree with them in this instance. They state they custom wind all of their springs having nothing in stock and once ordered there are no returns. I personally suspect they are just going through Moog's spring catalog and are ordering from Moog being nothing but a reseller.