If you want to use the body for the ground connection for the bat in the trunk, then there is no problem with that electrically providing the connection points are good.
But what is the benefit? You are adding weight because a THICK [ heavy ] copper cable is needed for the positive. If it is for weight transfer, is it worth it?
I don't have the wheel scales to prove it, but I believe it is. I grant a nice big cable has some weight, but it's low on the chassis, and pretty close to equally weighted on front and rear, maybe slightly biased to the rear.
And I think if you move a 27 from extreme front left of a (for example, rr) car, to extreme right rear, you have helped traction on the lift side of the rear end physics reaction, and I would bet at least a two percent weight distribution improvement. On my light car, it amounted to almost four percent. Anything I could do to help the 60/40 distribution I had, I was gonna do.