Your going to want (2) of the 2 1/2" clamps and you would need a 3" clamp if you want to clamp into your 3" exhuast, or you will have to use the reducer and another clamp/weld into the stock exhuast. Also a 3" 3 bolt gasket to replace the paper one that comes with it. When installing the band clamps on the y-pipe, make sure the crossover pipe is not resting on the transmission, tighten the crossover pipe to the front header and its a good idea to use some longer bolts on the crossover to front header, the ones that come with the kit are not that long and almost pull through when you tighten. Re-use stock manifold gaskets after inspecting them, they should be good, either that or get new stock ones.
When tightening/placing the band clamps on the y-pipe, make sure you when you tighten them that the bolts wont hit the firewall, I had them positioned so I could tighten them easier, but ended up having to shorten a couple bolts. I also tack welded the nuts on the clamps so I didn't need 2 wrenches under there. Tighten the **** out of them and use alot of rtv or your going to have to revisit them in a while. I think I was under there with a 2ft. pipe on the end of the wrench just tightening away...