OK. Personally... if I was you... I would find a donor Ranger or Explorer, 89-94, with a 4.0 and swap that in. Why? It'll bolt up to your transmission and the wiring changes are minimal. Or just replace the 2.9.
Why not use the 2.3?
Bellhousing, torque converter, flywheel and starter are all different
Your 87 wiring will be very difficult to graft onto an '89 truck
Motor mounts are much different, radiator is different
You could find an 89 up 4 cylinder harness but it won't work with the '87 engine
Personally I think swapping the 2.3 in is a huge waste of time unless you have a lot of free time on your hands and access to a substantial supply of parts. A 4.0 swap is just a lot easier and your truck is an ideal candidate for it, or just replace the 2.9.