I bought those from another slant six guy that had access to a lathe, the bolts are grade 8 hex head bolts drilled / tapped on the cap side to accept a threaded allen.
But a better solution is what I used on the cast crank motor that I built for the 68 Barracuda. For the windage tray mounts on that motor I used big block Chevy main studs that have windage tray mounting extensions formed into them. They fit fine, plenty of room for shims to get the windage tray to crank gap right. Be sure to check the screw in depth of the main studs so the cross drilled oil holes are not restricted.