Do you have the alternator harness from the '94? If so, it's plug and play, completely remove the '91 alternator harness and regulator and plug in the '94 one. The connectors are different colors but the interlock tab matches with its receptacle on the other half. That harness will likely have a slightly different set of connectors for the A/C compressor though. When ford went to R134 in '94 they added a high side pressure switch on the outlet of the compressor so you'll have to bypass that or use it. Since you're using the serpentine belt system you'll probably want to convert the A/C as well which involves getting the new style common hose assembly from the accumulator to the compressor to the inlet to the condenser and the newer style condenser which is more efficient. If you don't have/want A/C rockauto sells the Idler Pulley and bracket that goes in place of the compressor.
If you need to use your '91 harness for the 3G alternator, you have to remove the regulator and jump two of the pins in that connector I can't remember exactly which two but look for Homlessduck's recent thread about his '92 truck. It had the 3G upgrade done and someone posted what you need to do. Then you have to graft the 3G alternator plug in place of the old plug at the alt. You also really need to run a heavier gauge wire for the main charge wire from the alt since the new alt is rated for double the output of the old one. That really makes the '94 harness swap a better option if you have or can get it.