I'm auto crossing my car and I'm thinking of relocating my battery to where the rear passenger seat used to be. Is there any technical or regulatory reason I can't run the wires inside the car and through the firewall either with a grommet or through wall fitting? It seems like it would be safer and easier to work on. Would it work to connect the battery ground to the frame close to the battery then run a cable from the frame to the block in the engine compartment? Or better to run the ground cable all the way to the block through the firewall. Anyone done it this way? Any pics/tips?

 

Either way will work,I have the battery in my trunk on my Hot Rod,it has a kill switch in the cab,it goes from the starter to the switch,from the switch to the battery,but I used a heavier gauge cable.
grounded to the frame,in your case I would ground it to one of the seat belt bolts.