The voltage readings sound normal to me, with the engine running. I assume "excite wire" means the +12 V terminal of your "2 field terminal" alternator, which probably means your car is ~1972+ (didn't tell us). As I said, better to measure the voltage drop from BATT- to frame w/ engine running. 40 ohm sounds high, but much harder to measure resistance accurately.
A photo of your battery cable would help. In my cars, the BATT- cable goes to the engine block and nowhere else. Another braided cable goes from the engine block to the firewall. Can never have enough ground connections and maybe run a few new ground wires from the case of the Vreg, starter relay, and such to a common ground point if you don't trust a rusty sheet metal screw like the factory did.
