Lots of great info here.
As for moving the battery to the back, there isn't weight to be saved here at all but weight distribution is a huge deal for any kind of driving. Moving a big chunk of weight from the front to the back is incredibly helpful. Many modern cars come this way including the modern Challenger. Ford cheaped-out on my Mustang but at least they put it toward the rear of the engine bay on the passenger side.
I get that you can ground to the chassis and only run a positive cable to the front...but is that chassis ground reliable? It seems like there are a lot of welds and gaps between the trunk and the engine block.
The standard wisdom is to use a 0 or 1 gauge cable, correct? It seems like all of the battery relocation kits come with 2 gauge cable. I haven't done the math but I have to imagine that this would introduce a considerable amount of resistance.