Well, you are increasing your idle electrical loads with the addition of the electric pump. To me the major issue on proper alternator size is not necessarily the output rating (this is usually rated at 5000RPM shaft speed or higher), but how much output the unit has at engine idle speed. Alternators usually "cut-in" (start producing output current) at about 1100 shaft RPM. Near these speeds they produce much less than their max rating. Combine all low speed worst case loads, lights, pump, A/C, heater, stereo, etc, and you have a fair demand on the alternator at low speeds.
To close this discussion, the larger frame alternators (Delco 12SI), have better low speed output than smaller frame (Delco 10SI) alternators. Unfortunately, alternators are not described by their low speed output, but in general, you will be better off in your application with a 12SI. If I recall, these came in 84 Amp and 100 Amp. I would venture to say the 100 would still be better at low speed (cutin speed and max output are usually a trade-off).
I hope this helped rather than confused.......
Regards, Georg
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