I think it all depends upon how much you want to do to the motor. Drop in and go? Or take the time to go through it and replace the seals/gaskets etc.
If you are gonna crack it open for a look see, and put gaskets in and you know it is a good runner, I'd go for a pre hyper piston motor. If it hasn't been beat up and doesn't need new pistons and rods it will have forged parts already. In fact, the rotating assembly is good for more hp than the block will hold.
87-88 motor is Speed Density. Easy to convert to MAF, has a SLIGHTLY better factory cam(if it matters).
89-early 92 is MAF, late 92 lost the fordged pistons.
I will strongly suggest that what ever motor you get you do the following:
Rear main seal.
Valve cover gaskets.
Check the head gaskets as best you can, probably replace them and use good after market bolts/studs.
Water pump w/ new gaskets.
If you use the stock heads, expect to replace valve springs.
Keeps all the front accessory drive pieces you can, might come in handy.
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