67 Barracuda alternator with 2 FLD connections

The test specs in the '67 service manual are not the rating.

This is snip from a '69 or '70 FSM but the point is that its a maximum load test at 1250 engine rpm, 15 Volts.
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We can sometimes relate the test specs to the Chrysler alternator 'rating' by looking for the same description in other books.
This is from the dealer data book.
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source: The 1970 Hamtramck Registry Library Page (1965 - 1969)

Sometimes there is a part number for the ratings, such as service bulletin 66-78, which can be found here:
The 1970 Hamtramck Registry "1966 TSBs and Related Material Index" Page

If you got to get one from a parts store, see what they have. If they call it a 40 or 50 or 60, any one of them is probably fine. I wouldn't take one that was 'rated' over 60amps though.

I'll add that I had quite a few slant six cars and above idle speed the alternators always were able to power lights and wipers even with the heater fan on.