Both are wrong as there were three different 4.9 I6 motors.
4.9 L I6 1987–91 165 hp (123 kW) 275 lb·ft (373 N·m) EFI
4.9 L I6 1992–93 145 hp (108 kW) 265 lb·ft (359 N·m)
4.9 L I6 1994–96 150 hp (110 kW) 260 lb·ft (350 N·m)
Being a 1991 you should have the 165hp unless the truck was one of the last off the line and they used the 92 motor.
Try
https://www.fleet.ford.com/maintenan...ls/default.asp and see if it will give you the engine details.