The problem is usually lower in the block than the bolt threads, but I might try resealing the two bolts next to the
oil feed for the heads. Nothing more.
Just use the minimum torque spec, the tighter you make the bolts the better chance of opening up the crack.
They (the two bolts) are outside of the valve covers. Just try only that and see if it fixes it.
It’s a slim chance but you may be a lucky man…
What I would do - Remove the two bolts and clean out the threaded holes the very best you can, run a bottom tap down in there to remove any rust and debris, then use brake clean or some other volatile solvent, dry with lots of compressed air (the object here is to remove the
oil to allow the sealant to work).
Then add some wicking Loctite #290 (this will harden only in the crack) wait a few hrs, clean out the excess Loctite, choose your sealant and reassemble. Be advised that excess sealant can cause a hydraulic lock that may open the crack further, so using a thin anaerobic sealer like Loctite #545 take down the bolts slowly and cautiously giving the excess sealant time to migrate up around the threads. This should be done with your fingers so that you have a feel for the lock-up. After the bolts are seated on the heads torque to the low spec and wait 24 hrs for the sealant to cure.
*Note that even with the engine torn down to a bare block, the problem cannot be seen; it is down inside the water jacket. There are only two places on the FE engine that an
oil passage runs directly through the water jacket, the same on both sides.