If this were me...and I hope it never is, but I do own a 455 Buick block with a cracked starter bolt-hole...
1A. Get a carpenter's square, make sure the BUCKING FONEHEAD that welded that stud got it STRAIGHT in two planes--side/side, front/rear. If that stud isn't square to the block, you're done. Engine comes out, either gets repaired properly (not likely) or the block is scrap (very likely.)
1B. Use that square as a straightedge to verify that no part of the weld droops down into the starter-mounting area. You don't want the starter hanging-up on a weld booger so that it won't sit flush on the block.
2. Use a section of 3/8 ID tubing, or 3/8 plumbing pipe, or a bigass stack of 3/8 washers--whatever spacer seems reasonable and about the same length/depth as the starter casting--and torque a nut onto that stud the same as you'd be tightening a starter bolt. Welding cast iron is not easy, it's easy to create more cracks than you fill with weld-metal. Make sure the stud is properly anchored in the block.
3. There's no way to assure starter alignment since there's no "knurl on the starter bolt" for that hole. Even if the stud is square to the block, and secure in the block...the hole in the starter nose casting is going to be a sloppy fit on the stud. Be careful of the starter alignment; and beware that it could shift under load.
4. IF (big IF) that stud is genuinely straight up 'n' down in relation to the block, AND holds torque when tested, you're kinda on your own for getting a starter in place. You could cut-open the casting around the bolt hole that stud goes through, and hope for the best. You'll never turn that starter in as a "core" the next time the starter fails; which hopefully won't be for a decade or two. There may be other solutions--use your imagination.
I don't envy you. That's a hateful mess that you got left with.