Default [Q] lg e400 sd card default storage

i have a rooted lg optimus e400. the device has memory problem because the built in memory is separated into 2, system storage and internal storage. all downloaded apps is stored automatically on the system memory which is too low. i tried flashing a custom rom which merges system memory and internal memory, but the rom keeps on freezing and i decided to flash the stock rom. is there any way to use the stock rom and just set the sd card as default storage for downloaded apps? or maybe merge system memory with internal memory. or anyway to expand memory using whatever method? and if there is a way. could you give detailed instruction and links for the fix coz i'm kinda new at this. thanks