i have a friend who built a 406 sbc wuth new afr heads was very detailed in his build.
he as well has a leak around head studs so he removed the heads bought new gaskets and arp thread sealant and they still leaked. he then removed heads then sent them back to afr because he thought maybe it was his steam holes causing problems they said there was no problem so he put it back togeather and was still leaking then he was telling story at parts store and an old man told him to use km or kw or kd block sealer .. (im not sure on exact name.) he followed directions exactly and it worked for the last 3 years and he runs 10.30's on motor.
he swears by it.
but id see what engine builder does first..
goodluck .
if you use the block sealer by pas radiator when warming engine up or it may plug rad so im told..