Go to Full Tilt Boogie Racing Homepage and check out their rear bushing set for the IRS. that along with the irs brace and a set of 03-04 half-shafts and you'll be set. the bushing set is'nt cheap and if I'm not mistaken you'll have to upgrade the spiders in your pumpkin to 31 splines since your 99 has 28. however if you do the bushing kit alone you'll be out handling vettes and vipers. I would highly recommend the gear swap but in my opinion if your going 4:10s might as well go 4:30s. it's only a few hundred difference in RPMs. A lot of the people I know went with 4:10s and wish they'd gone 4:30s afterwards, our cars are heavy and the cubic inches are small so the more gear you throw at the car the better they respond. thesethanks guys!!! any other suggestions?
I have a full 04 IRS sitting in my garage and am getting ready to order the bushing kit myself, that with a set of 4:30s i already have and the Eibach sportline springs in my car now will make for a completely different car. the bushing kit is really involved and will take a couple days to do even if you're familiar with the set up thats why I sourced a seperate IRS and am doing it over the winter, that way come spring its drop the old one and install the upgraded one (sell old one to re-coop cost) im also upgrading shocks at the same time but am still up in the air as to which ones to get.
hope this helps, but again this is just one mans opinion.