I have been making mine one piece and have had no problems. I also don't even use headers gaskets just the red high heat RTV exhaust sealer. When it comes time to take the headers off, I can take all the bolts out and they are still stuck there . ( just takes a little wiggle and they snap off.

Flange material for carbon steel I always use COLD ROLL material it is allot stronger than hot roll formed steel because it is work hardened during the forming process. You can tell the difference between hot roll material and cold roll material quit easily .
Hot roll stock has mill scale ( that blue flaky skin ) and cold roll does not.

One piece hot roll flanges come completely flat , but once you weld your tube on the back side, it will put around a 1/8 overall back bend to the flange surface. This helps out considerably on sealing the big dodge two center exhaust ports.