Bear in mind that the K member, as example, might not be well grounded. It's bolted to the body, and may become rusted.
You need BIG (larger than no4) grounds with a trunk battery, because every inch of wire and body adds SOME resistance, therefore the only way to combat that is to use better connections and larger wire.
For ground terminals, at the very least sand/ grind down to metal and use antioxidant grease. Look to some of the big electrial supply outfits and get ground lugs with TWO bolt holes. We used to use "Burndy" ground lugs like these:
Another thing you might consider is some sort of heavier plate to give better surface area. Weld a 1/4" or 5/16 piece of plate or angle over a section of firewall or in the trunk, or even braze a piece of copper strap in there.
Make sure all connections are clean as can be and well made as can be.
Use a heavier duty battery to be sure you have enough CA, and you put it back there for traction, right?
Even on my front mount battery, in order to make sure the body and engine "are one" I have a short no4 starter cable going from the rear of the head to one of the master cylinder bolts.