Well it's grounded from the negative terminal straight to the cylinder head with 4 gauge battery cable. Im thinking it may be corroded where it is bolted on the head or the cable is broken inside the sheathing.
Correct, you have a known good battery/starter so it has to be in the large cables. Clean each terminal/connection (power and ground paths) & you should be good. also once I had a cheap made overseas cable that had a layer of green corrosion inbetween the strands and the crimped end terminal, took a bit to figure that one out. Oil/grease inbetween the starter/thin plat/block could interrupt that flow but is rare cuz there is alot of metal surface contact area with the starter wide flange down there. iirc you jumped it straight to the battery & still bad so start with the ground paths. EDIT & was the neg jumper cable clamped to the starter case?
