relocating battery to bed

I am going to relocate my battery to the bed since I got the new trailer fenders in front and it won't fit anymore. My question is, is it safe to run the power under the truck along the frame or should I run it through the cab. I plan on running 1 4 gauge wire for the positive. Also I have a 180 amp alt if that makes any difference on the wire size that I run to the bed. Thanks for any help in advance.