alternator replacement

[EKIMTNAL]

Have a 1988 lx 5.0 and with the headlights on and the heater fan on high my gauge drops like a stone. I have rebuilt the alt. with new brushes rectifier and tested it and it seems with in spec for a 75 amp alt. Just replaced the fan control switch and noticed the plastic connector was deformed by heat?
Is the factory alt too small? Should i be looking for bad grounds? Any help is appr.
Thanks

 

[87'GTstang]

You have to do two things:
1) Research mustangs and some of the issues to look for - in this case definitely charging
2) Search for a 3g alternator conversion to permanently fix the problem.