Delco is saying the duty cycle is 100%, but if you have 100 amps of load, you need 150 amp alternator to be safe and not over load it.
One thing that Towcat mentioned is amperage at and idle vs running rpm.
Some alternators do not charge much at an idle, ie 1G Ford alternator. Other alternators are designed for high loads at an idle such as
Leece Neville 7711JA which came on some of the Ford 7.3L IDI ambulances.
The Leece Neville 7711JA should deliver 135 amps at 3000 rpm (alternator speed, not engine) with a max amperage of 160 amps. So the idea is to size the pulley so at an idle the alternator is turning at 3000 rpm. However, you still have to verify that at max engine rpm the alternator is not above it's rated rpm.