I would actually be surprised if there weren't already rules for this out there somewhere....I'd like to redo all missile and LB autocannon hits. No more rolls on the cluster table. Margin of success of the to-hit roll is indexed to the number that hit. I'd want range to the target to modify this for the LB autocannon. I think I'd also want range to modify the number of missiles that hit, but could be talked out of it.
As an optional rule, the LB autocannon must divide the damage into two clusters at short range, 3 at medium range, and 5 at long range. If it does less damage than that, then each point is rolled separately. This would simulate the spreading of the sub-munitions nicely. Optional though, because it is more complicated.
Second optional rule - the LB autocannon gets no bonus at short range, +1 at medium range, and +2 at long range. Or maybe 0/2/4. So it gets easier to hit at longer ranges, but the damage is going down.
I don't like the concept of mech shotguns at all. Wouldn't bother me at all to remove them from the game entirely. But if I were to keep them, I'd probably just make LB a type of specialty ammo any autocannon can use. Not much different from flechette, after all.