Hello! I have done several 4.0L conversions.
I assume you have the 2.9L right now?
Email me and I will send you all the info you need.....
I no longer build the wiring harness for people, I just dont have the time but I can get you started..the wiring harness is the only things that is not a basically bolt in affair. You will want to use a 93-94 OHV 4.0L from either a Ranger or an Ex, the Aerostar 4.0L will work but thei wiring requires more modifications then it is worth....anywho without getting into all the details here and re-typing everything I have typed 600 times before email me so I can forward it to you....
Alec:
thats actually how I met Brett some 4 years ago, I had done my 4.0L, he caught wind of it, turned out we lived less then a mile from each other (no kidding) and so he came over to check it out, two months later he had a 4.0L in his BII and I had a lift with d35 and 8.8 on mine

the rest is history.....
88Vert:
I have seen one. Yes they did make a few 90 BII's with a factory 4.0L, but I have also seen many that were done later and people tried to pass them off as factory. No sirreee Its easy to tell the difference, the early 4.0L had a single 02 sensor, a map sensor, and the smaller under hood fuse panel if the 4.0L BII does not have these, it is anot a factory 4.0 BII.
Ford did some weird stuff, I have seen 89-90 BII's with D35 front ends while still keeping the MAS 2.9L and the 7.5 in the back, I have seen a 91 Explorer 4.0L with a rail mounted dual fuel pump setup (similar to a 2.9L), and it goes on and on, its almost like they just kept using parts till they ran out......