If the bolts are near the ends of the tubes, get in there first and weld in a reinforcement plate - a lump of 6mm etc welded in from the end would work.

If not then there are very few options really, you could drill a larger hole opposite the bolts so you can pass a socket and nut through to the front but that will only give you strength of the front 3mm section on the bolt, i doubt the hole will make any difference to the beam strength.