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)