I THINK I KILLED THE 340

I'm running MLS gaskets on a used block with new heads and no issues.
I don't know how they spec the bores though. Mine was advertised much smaller than the actual bore of the Gasket measured, so if that's a critical thing for you I'd call the Mfg for specs. If I had to guess, they spec the bore by what the block is bored to, not always what the Gasket measures, but that probably varies by Gasket Mfg too.