that will work yes! Just keep in mind no matter what V8 you choose there will be these area's to address:
-cooling, where can I fit a radiator/fan/shroud
-oil filter re-locate, the 5.0L oil filter is directly in your steering box, so you must re-locate it
- dual sump oil pan and pump
-engine mounts - several options
- exhaust manifolds, downpipe, etc - several options
-heater box w or w/o ac needs to be clearanced
- firewall tunnel clearance
-winring adaptation/insruments/gages/interface with the trucks systems (fuel, hvac, abs, dash, power distribution, lighting, cruise control and AC)
the 89 F-150 5.0L drivetrain is not going to impress you, in fact the 4.0L you have now makes similar power and torque
I suggest LOTS of research before you buy anything
Carb or EFI?
5 speed or auto?
2wd or 4x4?
whenever somebody asks about putting a SBF into a first gen explorer I will always recommend the Gen II Explorer 5.0L drivetrain
it has so much going for it
it is EFI, OBD-II, DIS, MAS, full roller GT-40 5.0L pushrod awesomeness and the transmission behind it is even better...when Ford put the 5.0L into the explorer they made it that much easier for you to put one in yours, the last 5.0L made was the 96-2001 Explorer 5.0L, the best one Ford ever built in a standard production vehicle