Hmmmm...hate to state the obvious, but you put the cart before the horse here. Esp. with the S&W kit, you need to build it around the seats that you choose. Now that I'm done stating the bloody obvious, the only things I can suggest:
1: Search the seat manufactures sites for narrow seats. Try the Scat ones.
2. Disassemble your rollcage and widen it.
When I did my main hoop, I actually stretched the hoop out quite a bit, just to give it that extra little bit. I also have my seats almost right up against the tunnel.
Hope that helps out a bit....
Tim