if I understand what you are saying that test is not good enough. The ground has to be able to handle the full current of the load just like it was a wire. OEM systems traditionally have very poor and very small grounds between battery/ engine and the frame/ body. A good example is installing winches, snowplow lifts, and big lights onf 4x4's. The never had adequate ground ties for any of that.
I would run at least a no4 "starter' cable, "eyelet to eyelet" type cable. You can stack connections on a side post but I would not stack more than two
if you search around there are various adapters sold for adding connections to side terminal batteries.
