Take the rocker cover to a good welder he can fill it and drill and tap a new hole. I did this with my front head when the exhaust stud pulled out.

 

It's not running right now. Very hard to start, rear cylinder exhaust sounds different than front and has a pop in its rhythm with smoke. Compression test shows 120 for front and 75 for rear, if I try a few times it sometimes hits 120, but not consistent like front cylinder.

Joe

 

Pull your rear cover off & roll the engine over with both plugs out. Pay attention to the valve movement. i think you have an intake sticking open for some reason.

 

Well, the good news is that The MoCo sells them individually, not as a set. Which will save some $$$. Also, the lower is the least likely to be warped, so a used one may be a good option.

 

It ends up you can not put longer bolts in the primary side of rocker cover, they hit the valve spring. After getting it started it needed some carburetor adjustments and then it was fine. Used red locktite and it seems to be holding, no more oil leak, for now!