You can use this stuff that I like called QuickSteel. It's a putty that becomes semi permanent.
Better than that, though, would be to get an exhaust reducer and just use some lawnmower exhaust gasket material (Ace of any local lawnmower shop should carry it. It comes in varying thicknesses). Clamp the properly sized reducer onto your factory header and then attach the other end to your slip on. That'll work and you won't have to find a specially sized exhaust coupler gasket.
They look like this:
EDIT: I would not completely remove the rear fender bars. If that's the look you are going for, and you know what you are doing, that's one thing. But retaining those and just reducing the profile of the rear fender itself will still give you look that you want plus some protection from the elements. Until you have never ridden on wet pavement without a rear fender, you won't fully appreciate why I am saying this.
(rooster tail)
Not only that, you're resale value will stay higher if you don't mess with the frame.