FWIW, the Baumann (BE Controls) kit is very similar to the J-mod. It has you drill a few holes to a larger size depending on your desired firmness level and also comes with the springs if I remember correctly. I did it back in 2005 or 2006, so I can't remember much of the specifics. The instructions were pretty straight-forward and were easy to follow.
I've been very happy with it. I set it to "level 5" and it's really firm when you get on the throttle and will normally chirp the tires on anything over about 1/2 throttle. Light throttle is still pretty firm, but I don't mind it at all. My wife hates it, but she never drives it, so it's ok. lol.