well, i know that the core supports are different from 96-98 and 99-00, but you can get away with that. you have to trim the new 99-00 front bumper in a couple of areas.
as for your setup, the 4dr front ends are different than the 2 door front ends. so i believe youre just going to have to get a 4dr 99-00 front. you could get a front lip, and im sure they make knock off si grilles for it so you could get the same look as an si.