MM:
Typically the fuselage doublers run aft to about the highpoint of the wing - that is good practice for IC or electric. The next key is to distribute the motor mount load into the doublers. Long beams do a good job for IC airplanes, for electric you should integrate or reinforce the motor mounting plate to the doublers & surrounding structures. One potential problem when converting kits is that the doublers often stop 1" or so short of the nose ring. If you are using a FRONT mounted motor you will likely want to make new doublers at least as far as the motor mount.Rear mounts usually end up somewhat forward of where the F1 normally goes, so the install is usually friendlier.
The doublers are more than adequate to perform as the sides of the battery box, Assuming that the doublers go past the wing LE as described above then you are in good shape. The battery tray(s) of course need to be secured to the doublers and (preferably) the FI & F2 bulkheads.
For further explanation, I attached a couple pix of the new conversion I designed for the Brodak Gypsy. The conversion features new doublers with tab/slot design to integrate the front-mounted motor mount, F1, F2 and the battery tray. This is very robust AND lighter than the IC installation - not a bad trade!