1998+ rangers shared alot of parts with explorers of the same years. Especially Edge models which came with the same tortion bar suspension and the 302 from the explorer will bolt in that model. The 2wd model rangers come with coil spring front ends, so a motor mount adapter plate is required, which is readily available. After that the rest is a bolt in as well. Finally if your friend wants to keep fuel injection all of the explorer wiring is plug and play as well. All he would need to do is switch one wire on the back of the guage cluster to adapt the stock tach to the new v8, and make sure to use wiring from an early v8 explorer that was not equiped with the PATS system (computer chip in the key). You can use the PATS sytem if you like but its a major PITA to get it programmed and workind correctly.
I realize that all of this info is based on 302's and not 351's but all should be the same except for the headers because of the taller deck. I dont recall anyone ever installing a 351 in a 1998 and newer ranger yet.
Just wanted to add that as far as headers go, the only company that makes a header for the explorer is torquemonster. Ford racing made a shorty but stopped production some time ago. Im also not sure if it was for the explorers equipped with gt40 or gt40p heads or both :smt102
I bring up Explorer headers because some of the guys with 351's in 1997 and older trucks use a 302 swap header and modify the steering a little to make it work. So using an explorer header on a newer truck with the similar frame may be a good starting point when going 351.