This is a pretty common FE problem.
It can usually be attributed to a cracked block.
There are a couple of “fixes”
Gessford has published the AERA (automotive engine rebuilders association) bulletins regarding the cause and repair of this problem.
Here;
Gessford.com
(I have done the repair in the vehicle, a gamble, but it was just a car with a 390 and the risk of failure was not as critical as an expensive performance engine)