The Mr. Gasket white will last for years, BTDT. I've carefully pried them off to reuse them when they had to come off. Cut the flanges like I said and they cannot blow out any longer as they will each then be firmly pinched in place. You guys should try it, when headering cars and trucks at the garage we started doing it after much grief with bolts backing off and header leaks out the wazoo often in days. Cut the flanges to let each pipe settle in slightly different like it wants and all that goes away. I used to never retighten header bolts at all after doing it. The AMC big tube header was known for cracking pipes at the flange, the procedure stopped that as well. The guys who grind the flanges 'flat' (a joke!) after welding are bloody butchers and no way is that overall flange length anywhere NEAR being dead flat. I've found that Felpro exhaust gaskets generally allow way too many leaks, they are bigger holes to cover the bigger flat cast iron manifolds have, we almost never could use them on headers back in the day as they would not line up without overhang to leak in ten places. They are too thin and do not 'fill' irregularities in the flat as well either. The thicker the better there but you have to pinch it all the way around or blowout city.
Look close at the gasket you pull off the header, 90% of them do not pinch at all about half the places they should, cut flanges and you get 100% pinch all the way around each port if you've dressed the weld down a bit more level with a flat file. It allows each pipe to pull up slightly different from its' neighbor to seal better.
I never plugged the holes in mine and no leaks at all there ever. Hooker welded a plug flat there to cover it.