If the bolt isn't broken inside, consider yourself lucky. Short of getting a thick aluminum washer, taking your time sanding it and getting it just right to fit in there not much you can do except change the block or find a great welder, the kind that is portable and drives to you. He can weld that up, grind it flush and redo the hole to where it'd be hard to spot it ever had an issue. On the other hand, a bad welder will finish ruining the block! We have portable stairs for our 50 seater commuter, had it welded twice by two different welding shops, repairs never lasted but welds looked crappy. Finally took it to the right shop, they fixed it and fixed it right, these guys were pro's in Aluminum welding. That might be your best permanent fix. 480 an hour, should be an easy 30 minute weld. Will take more in grinding it smooth and making sure the metal doesn't get in anywhere.
If you Opt for the washer shim, use Lock Tite on the bolt to make sure it doesn't go loose and it all goes down the tubes quick!