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