15/64" is the largest I can drill the hole and not cut into the Head Bolt Boss on my 9799914 1970 400 Block.
I drilled the heads to 15/64" and used a 45° carbide countersink to bevel the mouth of the opening (in the head only) out to match the 1/4" hole in the head gasket. The countersink is about .015" deep.
Since water flow is upward into the heads, this should promote good flow characteristics and maintain structural integrity of the block and head around the bolt boss.
I hope this information is helpful...Robert
