I have seen quite a few of the later one piece rear seal blocks stress crack above the lifter bores. Usually toward the rear and up a couple inches. Running at an angle across the surface.
I have drilled the ends of the cracks, V-eed these cracks out, heated the block to about 500 degrees and welded the blocks. But, this is not something most shops can do. Also a bit spendy if you have a blacksmith do it.
Another block would be cheaper, unless you really need the matching numbers