No, it will not drop in and run. You can take the 88 head, if its good, and put it on the 91 block. I did something similar by converting my 91 to a 4 plug head. Since you have the 88 wiring, yours will be easier. Put the 91 block in, drop in the distributor, put the head on, and you will be good!
1991 Mustang LX 2.3L 5 Speed convert
1989 Ranger 4X4 2.5L 5 Speed solid Dana 30 front axle, 4.5" suspension lift, 3" body. ( blown up....)
Entire turbo kit waiting to be used again.