How Much Work is this: Converter

What alternator negative?
Are you running an isolated field alternator on the 69 and older single wire harness? Then grounding the second brush is correct, anywhere, so long as the circuit returns to the battery negative.
The small round-back isolated field alternator looks the same as the older round backs. The only difference is that the isolated field has TWO connectors for regulator wires, whereas the older style has only one connection for the regulator.
The regulators between them are also different; the older one is an electro-mechanical device,painted black, about 3.5 x 1.25 x 1.75 tall,and with screw-on terminals. The later style is all electronic, about 2.5 x 3.5 x 1/2 inch thick with just one weatherproof 2-terminal push-on and lock,connector on it.All dimensions are approximate as I'm going from memory. But you cannot mistake them, the late one is pretty flat.
You can run the isolated field alternator on either regulator,but you cannot run the old-style alt on the new style reg. Well you could but there would be some rewiring involved,lol.Both regulators must be grounded to the chassis. Both alternators are grounded to the engine by the bracketry, and from thence to the battery via the ground cable at the driver's side head. If the starter works, then I think it's safe to say,you can assume the alternator case is also grounded.
The large gauge output cable has not been referenced in the above discussion.