You may have most of the noise coming from a leak where the exhaust pipe meets the manifold. There's a big doughnut-shaped gasket that belongs there and you have to get the old nuts off without breaking the studs in the manifold. A nut splitter or oxy-acetylene torch are really handy for that.
If the noise is coming from exhaust gas leaking at your new joints, there is some cement type goop that you can smear on the inside pipe before you slide the outside pipe on. Then you clamp the pipes together and the goop sets up as soon as the pipe gets hot.