I had them on my 340 with headers in my dart.
They looked like they sealed like crazy, but due to a timing chain problem i had to remove the exhaust, so i never seen if they leaked or not.
I have a busted off stud on the back of one of the heads and they still sealed great.
The heads are going to get changed out for milled 360's but the Remflex gaskets are everything they say they are.