anyone know stock '68 alternator amp output

It is a crap shoot whether a parts store alternator has the matching rotor and stator. Of if they even did anything more than clean the exterior and spray it with some paint. In some ways better off with a junk yard find and replace the bearings and brushes as needed. Or you can buy a real numbers matching restoration, but of course those carry a premium price.

To amplify on what was stated earlier in this thread, the test specs and the labeled (or claimed) amp rating never match because they are done at based on different points in the performance curves. Power out is controlled by the guts (rotor and stator), rpm and voltage to the field windings. Amperage output is based on demand. So to reiterate a point made earlier in this thread, a higher output alternator will not neccessarily place more load on connections. It could put more amps through a poor one in some circumstances.

1968 Plymouth Alternator specs from page 8-96 of the Service Manual.

Field Current draw at 12 V: 2.38 to 2.75 amps (max)
Current Output:
Standard w/ slant 6, 26 Amps
Standard all others, 34.5 Amps
Heavy Duty or with A/C, 44 Amps
Special Equipment, 51 Amps

All above are allowed plus minus 3 Amps for temperature variation.
Current Output above is measured at the alternator with the engine at 1250 RPM and 15 Volts. If current is measured at Battery, output is expected to be approximately 5 Amps less. Voltage is controlled by a variable load (carbon pile) across the battery.
The outputs are not the maximum current.
Change the rpm, or change the voltage to the field and the amps out can be changed.

The newer style alternators do in general seem to be slightly more efficient.
The 'square backs' are easier to service.
It is unclear why Chrysler chose to switch from regulating the hot side of the field windings to the ground side when they switched to an electronic voltage regulator in 1970. But that is the entire difference between the 'single field' and 'two field' systems. As far as performance goes, either approach works equally well and can be done electronically.
Fairly in-depth discussion at ST, including a link to 70s or 80s Dodge shop manual * which actually shows the factory test outputs by nominal amp rating (instead of the descriptions such as "Heavy Duty").
How Alternators are "Upgraded" • Speed Talk

At some point, Mopar Performance found their electronic regulators for the single field wire systems were not surviving. Their solution apparently was to say it was a race only item.
Nowadays many replacement regulars are electronic guts instead of points. The question then is of quality. For that reason, when my last p-part died I switched to a NAPA Echilen equivalent VR1001 (listed for Studabakers) and just picked up a Standard VR 128 as backup. Both made in the USA and have a good reputation.

One of the worst situation for an ammeter circuit like ours (shunt at the meter in the dash) is using the alternator to recharge a fully discharged battery while driving.

*Hotlink of shop manual image from photobucket. Original posted by KThaxton in the Ramcharger truck forums. http://i438.photobucket.com/albums/qq109/kthaxton_photo/scan-2.jpg