Duster should have been a DEMON! Cranking issues!

I'd run a ground from the battery to the engine block, 4 ga. or larger. then from the block to the body. I've had a bad starter do that also. If you have another you might want to swap it out and try that. You need a larger battery feed then 8 gauge, that is only good for 40 amps. I bet that hot lead gets very warm!