OR put a piece of tube inside the collector that *just* fits inside it, and weld it into place such that ~3/4" of it sticks out downstream of the flange. On the mating side have a piece of tubing expanded to be a slip-fit over the that tube that you welded into the collector. Open up the ID of the mating flange to just fit over the expanded tube and weld the flange in place flush to one end of the expanded tube. Then put the new head-pipe inside the expanded tube and weld them together at the other edge of the expanded tubing. When you're done you'll have created a connection that looks like this:
Now a small bead of high temp silicone is the most that you'll need to seal that joint. I've been running joints like this for 25+ years with no leaks and no failures; and most of them are assembled with no sealant.
The first system that I built this way was under Patch. I built it because the header collector gasket failed and was making me very, very sleepy on a morning drive home from the desert races. Couldn't hear it leaking, but it was and it could have been deadly.
The system built for the Valiant shows the details best:
Using these joints I build the exhaust systems to be easily removed in sections so that If I need access to something that the exhaust is in the way of I can simply unbolt the exhaust and remove it.
ETA: I used to buy those flanges and then machine or grind out the ID of each downstream flange to fit over the expanded tubing. Now I just have SendCutSend cut them out already oversized.