I used Detroit-Eaton Spring. I did mine twice to get the correct height. Others I know have used Espo , MU and other vendors, who also did their springs at least twice. While I know customer service from Detroit-Eaton and Espo was very good, it still required doing it twice. Of course you can get pretty quick at this job. If i had to do springs again, I would take it to a local spring shop, tell them the height I want, and let them do it.
Make sure to see if you can remove the front eyebolts on both sides before going far with your project. While the nut will come off, it is not uncommon to have the eyebolt on one side seized to the inside metal sleeve (hardened steel of course). When this happens it can take a day or a day and a half to do one side, instead of an hour.
Steve