If you are serious about a rebuild, you need not design it for extra packs to be added. Designing it to accept one known configuration and optimising is better for that kind of machine. Now we are allowed 12S lipos you could design it for two 2650mAh 6 Cell packs (Wired in series and used as a pair) knowing that it will always work and and always fit.
If you want to be able to change it, make the head adjustable so that you can switch between something that cuts, something that clobbers and something that lifts (Or anything really). That is better far better (IMO) than messing with your internals.