What's the difference? it seems next to impossible to find a used kit that's for 99 only, so can one easily modify it to work on a 99 maxima?

According to stillen, the SC bracket is slightly different...something about small changes in the timing chain cover in 99's. However, this hasn't been confirmed by anyone else.

I bought a new 99 kit...figured it wasn't worth the hassle not to.

When I looked into it, it was not easy. Simply not worth trying to retrofit one on. Might as well spend that money for a new one, unless someone with a 99 ever sells one. Probably wont go cheap though, since there are several members looking to get a used 99. (or were looking)