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Seriously the OHV changed very little from 91 to 2001, you can bolt any OHV long block (block and heads) into your early 91-94 truck, just use your original accessories, intakes, manifolds, etc
I have done this several times, now there are some things to pay attention to, like the CPS, EGR, flywheel/flexplate, etc... but you can use the block and heads from lower mileage later model Explorer's and Rangers
the auto transmission, not so much they are computer controlled, so swapping to later model auto trans = computer swap = more complicated.
The SOHC 4.0L can also be used, but thats a whole different thread and a MAJOR conversion
I think I have 3 threads on this forum detailing my 4.0L OHV conversions... keeping old trucks going using junk yard engines...thats pretty much how I have kept our entire fleet of Fords running at my house no car payment 15+ years for me