Fairly common problem with 6.x engines.
Some options are thread inserts (commonly known as a Helicoil) and Lock n stitch inserts.
Welding is an option, but it's very difficult to do properly with the engine in the truck. IE: heat the block, specific welding wire, control the cooling rate, etc. The welder really has to know his stuff to weld the cast block and most won't want lay on their back in the welding slag spray and attempt it in the truck. Then there's also the issue of drilling the holes square while in situ.
I'd get it fixed sooner rather than later if possible too.
6.X's are weak in the starter mount areas to start with and the outer bolt hole has been known to break the whole "ear" right off the block or punch out the side of the bolt hole. Very bad, usually requires block replacement to fix. Has to do with how GM cast the blocks. Not enough "meat" in the outer bolt area.
If your starter is "banging around" in loose bolt holes, the block ear breaking out is a very real possibility.
The thread inserts and lock n stitch are also a gamble due to the block lacking "meat" in the ear. Not a whole lot for the loch n stitch to grab and reinforce in the first place.
Check to see if there's a bracket on the "front" of the starter (part facing the front of the vehicle) that bolts to the block. GM added this bracket to take some strain off the mounting hole area. Without this bracket, you are putting more stress on the area than necessary/intended. The bracket was Gm's way of compensating for the design weakness rather than fix the blocks during the production run...
Most mechanics that replace these starters don't bother to replace the bracket, it just gets tossed. Bad tech, bad.....
No biggie to replace if it's missing though. A simple piece of 1/4" angle and a drill press will make a perfect low buck replacement.
Someone at GM deserves a good slap in the noodle for scrimping on the starter mounting area on these blocks, as well as other issues in the design with cracking.
I wish I could have offered better news for ya.