I found out the hard way BUT....
You need to take out the front cross member and front engine motor mount as well as the rad fan in order to have enough clearance to get it in. Stock manifold goes out through the top, 4-1 needs to go in from below. Then, the bracket on the header (on what would have been the secondary pipe on a 4-2-1) will bump into the oil pan before you can get the header up and around the head studs. So, you need to trim a bit of that bracket to slip the header onto the head studs then custom fabricate a bracket to make the factory hanger meet up to the hanger on the header because its in a different position.
Aside from that it bolts right in! I was told that the 4-2-1 does not have much of a power advantage over the stock Si manifold and that the 4-1 would be the way to go. I haven't tested it yet on a dyno, but that'll be soon. As for clearance, I'm running the car in Solo II in the STS category and the car is lowered with the tires (205/50-15) flush with the fenders, or 4-1/2" to the jacking points. I haven't had any clearance issues.