Maybe put a longer bolt back through in opposite direction.
Since you have no access to the bolt head to keep it from spinning when tightening, you might try this: Before inst. the longer bolt, grind opposing 1/4" flat spots at the end of the new bolt, that way you can hold with Vicegrips untill all is tight.
Also, what type nuts, threads, etc. do the other 3 bolts use? They should all be the same.