I think I figure out a way to built an A123 5s2p pack (16.5V and 4600mAh) for no-custom(brushless) E-maxx 3905. It has an H shape. In the middle has a cell over the other and it seats exactly between titan550 and EVX2. If my calculations are correct it will fit perfectly to the E-Maxx's battery compartments and in the middle of the body, without any modification.

Emaxx battery compartment: L*W*H:157*50*25 mm
Titan - EVX2 area: L*W*H: 40*70*... mm
A123 26650 spec L*D: 67*26 mm

I still dont have A123 cells to test it, so if someone find it interesting and it try it, let me know