Relocating the battery to the trunk is not a cheap modification to do properly. You should be using #1/0 cable for BOTH 12V and ground. A dedicated ground should go from the trunk to the engine block, then to the chassis. Some people will tell you that they only grounded the battery to the frame in the back of the car and everything is fine, but that really is not the proper way to do it. Grounds are just as important as hot legs, especially on an EFI vehicle.
A battery relocation is not worth it unless it's something you really really want to do.