Alternator/Gauge reading question

Have a question regarding the charging system on an 89 LX 5.0L.
At 1500-2000 rpm( with accessories running) my gauge reads 12-14 volts approx- mostly at 12 but at idle and with accessories running such as windshield wipers,heater, headlights the gauge drops down to 8-10 .I also noticed the wipers slowing down at idle.I recently replaced my alternator that was making a slight buzzing/humming sound( and killing the old battery) with an alternator off a 88 2.3L(slightly used).I was told at the parts store the alternators are the same on the 5.0L and 2.3L so i figured id be able to use it.I installed a new Interstate battery appropriate for the car and have no corrosion on any wires.So...is it normal for the guage reading to drop to 8-10 V at idle or should the alternator keep it near 12-14V.A new alternator is close to $300.00.At this point im assuming that ill need one.What should the guage be reading with a properly functioning system thru the rpm ranges?Thanks alot for any feedback.

alternator

Thanks for responses and info,i will certainly check into 94-95 alt.
I wondered if pulley sizes were diff from the alt on the 2.3 and the 5.0 so dbl checked .The 5.0L pulley is smaller by about a half inch approx in diameter so maybe its as simple as that.Will try n swap them around first and see if it keeps the voltage up.I didnt think of it before making this post but i took my 93 LX out for a drive and watched the voltage guage ( amazing how i never paid it much attention before ) With everything running full tilt high beam on, wipers at full ,heater blowing full, rear defog and stereo going the needle fluctuated slightly from 14 on the dash guage and never dropped, either at idle or thru to highway speed.Thanks again for the advice, this forum has helped me out many times and i appreciate the time you all take to reply to questions.