There are DC grounds, lightning grounds, and bonding wires, and which ones to set up, how and why, can be long discussions and choices. (Check around online.) I'd expect the cable from your backstay to the keel is a lightning ground, which you'd want to keep separate from any others.
The fuel tank, and fuel filler hose (there's normally a ground wire from the filler cap), would be grounded to dissipate any static charge, so it can't spark and flash the fuel. I'm not sure what's 'code' on that but would expect both to go to the engine block and the DC power ground.
"Both of these grounds are in bad shape" Stick to fully tinned wire, it is worth the extra price. You can often get fully tinned "type 3 machine wire" which is very similar to battery cable, at a better price.