If you are looking to run 2 3s packs in a series for 6s total, I'd look to the Feigao 11XL as it will be more suited to 6s with its Kv of 1577.
I'm not sure what ESC you have here, but are you sure it has brakes and/or reverse? Most chinese ESC's are forward only and meant for air use. If this is the case, you'll most likely need to maintain the mechanical brakes and obtain a PiStix to properly run the ESC.
As far as the conversion, you say you want cheap, but quality is cheap then as well. Mounts bend and shit breaks.
Why not support the man running this site. Mike offers the best quality mounts around and has trays and conversion kits for the RC8B.
FULL CHASSIS KIT
Or
Mount only