So I remeber a post long ago I think it was here about some tricks to doing this quickly. I am building my uncle a set of headers for a 440 in his car. I am thinking of using a deadblow and flatining (sp) out the tubes till they fit then tack weld them then hammer them to fit on the inside. Does this sound about right?

Yep

Heres what I did, gently knocking in
to round tube, tapping the sides with a flat body hammer to flatten as you go. Takes about two minutes to square a tube up to flange hole.

Another pic

seen it done pretty simple
take some flat stock cut a hole same size your hedder flange is take your tube tap in the sides enuff to get to fit in the square hole slide tube into the square drice a tappered wedge (pyrmid) inside the tube
makes a perfict fit
least to the jig you made

I make a plug the size of the flange opening with a
tapered lead on it ... I weld that to a solid rod
(about 3/4 to 1") then drive that into the pipe
and tap the tubing to fit OR... this is the way I
did it at work.. same tapered plug then weld it to
1" fine thread bolt, then screw that into the flaring
section of our Huth tube bender and ram the plug
in... I could have all 8 done in 5 minutes... so
if you got a exhaust shop near you do what I said
and take you plug and tube to them only a few bucks
(also lube the plug with grease so it slides easy)