Dowel Pin Clearance
Another contributing factor to a "Blown Head Gasket" is dowel pin clearance - or lack of clearance, which can occur when a cylinder head is milled too much.
This is an often-overlooked area and can cause major problems. The dowel pin, which is used to align the block with the cylinder head, can cause the head to not meet flush with the head gasket and the block. Basically the dowel pins are bottoming out.
The best way to check this is to measure the depth of the dowel pin-receiving hole in the head and the height of the dowel pin. It is recommended you have an extra .050" depth in the head-receiving hole. Drilling deeper into the receiving holes can do this, but be careful not to drill too much.
Dowel Pin in Block
Dowel Pin Hole in Cylinder