I used to have this problem with a 200CI 6-cyl in my '65 (back before it grew two more cylinders). I had Clifford headers on it, and the header gasket between the third and fourth cylinder used to blow out consistently, leaving my otherwise mellow-OK sounding 6-cyl sounding like a thrashed Vanagon with a blown-out head gasket. I'm sure you can relate.
Mr. Gasket makes (or used to make) header gaskets for this engine. They're the 'fiber' type, without a metallic core. Summit has some made by Hooker under their P/N HOK-10827HKR that appear to be similar.
Order that gasket, and soak it overnight in a tub of water. Don't use your wife's turkey pan. Trust me on this. In the morning, disassemble the headers and clean them and the cylinder head. Check your header flange, as noted above, to make sure it's not really, really warped (if it is, this isn't going to work, and neither is anything else). Make sure the rest of the engine is ready to run. Take your dripping header gasket and install it. Bolt up your headers just like normal, but don't dawdle here - you want them to remain wet. Tighten your header bolts and start the engine. Let the engine run for long enough to evaporate all of the moisture from the gasket - at least 20 minutes or so. Voila! Exhaust leak solved. The wet gaskets will conform to whatever imperfection was causing your leak in the first place (which is still there, by the way!)
This was the only thing that worked for me on the 6-cyl after trying dozens of combinations of gaskets, sealers, etc. The wet-the-gasket trick then sealed the exhaust up for the next dozen years (until I got tired of 120bhp). Hope that helps