anyone know stock '68 alternator amp output

Per the test method, that is what the FSM says. Read it carefully.

Just bolt any round-back on,and you are golden. The regulator will take care of everything. If a dual-field alternator is used,then the second terminal is just to be grounded.
As to square-backs, anything that bolts right on will also work, but the higher output ones, IIRC have their own required regulators.
Pre-1970 alternators were single field, and used mechanical regulators. I highly recommend to get rid of that little trouble maker. The dual-field system is practically trouble free, and the retro-fit is easy. You just add one wire and a special plug for the electronic regulator;easy-peasy. Kits are available from Mopar.