Assuming the alternator pictured in post # 2 is the one off the 1998 Gold Wing....!!
Will it fit a 91 1500?
True it will fit but you would need to make a little change.
The alternator pictured in post # 2 is not an OEM alternator. It is an aftermarket alternator.
The "exciter wire" on the alternator in post # 2 is permanently attached to the back of the alternator with a connector that plugs in behind the left fairing inner panel.
You would need to remove the left fairing inner panel to plug in the "exciter wire".
OEM......The "exciter wire" plugs in to the back of the alternator
Also note the color of the "stator coil", black on aftermarket and green on OEM alternators.
You would have two options:
1. Remove the panel and plug in the "exciter wire".
2. Cut and splice the "exciter wire".
See attached OEM alternator..........!!