Just curious, what is the right way to convert a 2 field roundback alternator to a 1 field so it can be used on a stock 68 small block?

Thanks,
-Jon

Just ground one of the leads.

You could pirate a grounded brush holder from a single field terminal alt for a hidden/cleaner look or take off your insulated brush holder & rework it a bit so the screw grounds it

depending of which roundback alt you actually have, some have both provisions and you can get the unisolation kind brush and remove the isolated one oppositte to it ( attached an example of the alt with both provisions ), but IF NOT, simply replace the isolation washer on one of the brushes screws for a metallic one and will be done. No need for extra wires hanging around making that

for safety, cut the prong on it, to be safer from a wron brush field wiring