There is a couple of recent issues of Kit Car where Dean has some good info about setting up his 40. You must have read those. I believe Paved Track Stock Car Technology says the roll center should be between 2 and 5" as you said. Although you might think a lower RC is better I think that below ground level has a gyroscopic effect on the car. One of the keys to roll center height is getting the lower control arms level at ride height isn't it? Heidts makes MII suspension components and has a pretty good explanation and diagrams on their website of how they should be set up and why it works. They also have awards for their 2" drop spindle you mentioned. See
http://www.heidts.com/pdf/Understanding_IFS.pdf
The pictures suggestion would sure help, maybe there's something obvious like the rack is mounted on the wrong side of its brackets or something silly like that. I don't know where your manufacturer is at right now but I'm sure they would have some technical support/advice available for this dilemma.