What's the easiest way to make a round header tube fit into a square hole flange?

Hope this helps, it's actually part of a great header building series.

http://m.youtube.com/watch?v=F-2SX7bUIlY

I always make a plug with the port shape .. I weld
either a handle on it so I can drive it into the pipe
or weld a 1" fine thread bolt on it and take it to
the local muffler shop so they can press it in on
their Huth pipe bender(that way only takes a couple
of minutes to do all 8).. mark the tube for the top
center of the port with the right rotation... I have
plugs in the drawer for all the different headers
I've made

I made a tapered plug that started out round then went to my port shape, I welded a peice of tubing on the end to fit on my air hammer. The air hammer drives it in and a small tap or two on the sides and it's ready to weld.

Got any pictures ?

And with the tubing

The finished tubes