[Tom Goldman]

I never had any luck with the white PTFE type thread sealers on stock BBC blocks .
Most of the blocks have pitting in the bolt holes from years of street use and as was mentioned can be further enlarged by chasing the threads with a tap.
The best thing I've found for sealing the studs is pipe thread sealer made for gas pipes. ...My brand of choice is Leak Lock from Highside Chemicals. Its blue in color and is a hardening type of sealant.
I've used it for 20 years on my old 468 spare engine and have never had any leak problems.
As you said in your last post ,make sure there is adequate clamping , I 've had to double washer some engines that have had the heads angle milled.