I like Steve's idea. Use the cutoff rivets to align and weld. Welding it back into place will hold. FWIW, my bumper/rear crossmember are held in place with welded rivets.
If you're set on using bolts, I can think of one way that I've done myself:
When I needed to replace my XJ rear shocks, 2 of the 4 the bolts snapped off from the captured nuts. I googled and found a video on how to insert bolts into the hole (after removing the captured nut with my air hammer) from the top as there was just enough room to do this above the shock mount.
So, once you knock the cutoff rivets out of the hole, you're pretty much committed, therefore maybe practice this on another blind hole to see if you like it.
Take a small wire (I used leftover copper wire from a florescent light ballast replacement and stripped the insulation off) and tightly wrap it around in the bolt threads (start in the middle and work to the end of the threads). Now you have a "bolt on a string" and can fish the copper wire into the hole and pull the bolt up into the hole. You may have to reverse fish the copper wire down into the boxed part of your frame until you can get it to exit out where you can grab it and then wrap the bolt.
For my XJ shocks, the bolts would hang down through the hole; gravity was working in my favor. Here, if you did this, letting go of the bolt means starting over again. So push the bolt against the frame hole (bind it so it doesn't move), remove the wire carefully and thread a nut on to capture the bolt.
That gives you something to vise grip and pull to spot weld the bolt (you did say you were going to weld, right?). Since I'm not a welder, I don't know what the heat would do to the bolt ( I think I'd be using Grade 8 here). Or have the shock mount in place to thread the copper fishing line through in order to finish the bolt on mod.
A lock washer on the bottom (next to the head) may help lock the bolt during tightening but may not help if you intend to spot weld the bolt.
