Four bolt mains

There is a reason why the outer bolts are 3/8 on those caps (and they are offset) and on the T/A, X and R blocks the caps have 1/2 inch in line bolts that aren't splayed.

When you stick that bolt out there like that, you better have the main caps on and torqued when you hone the block or it will pull the bottom of the bores out of round (caps should be on an torqued anyway, but I've seen many guys that leave them off).

There isn't much materiel out there. Any hole you put there will eventually fracture. Always.

If you keep the RPM's down, and keep it out of detonation it will probably live. But...the block is inherently weak right there, and the threads and hole there are just as likely to form a fracture as not.

That cap with studs and cutting off the outboard bolts would be better than using the outboard bolts.