the guy that put the 6.0 in also built that thing to the hilt and couldnt keep it together. im pretty sure he has since sold it because it would break almost every time it went down the track. a stock motor would definitely be different.
in my opinion your best bet is probably the 4bt. I have seen a 6 cylinder cummins swapped in to a ranger, but im pretty sure he had to make some serious firewall modifications.
as far as the 6.4 aspect, i cant imagine that even fitting without extensive frame and sheetmetal work. the motor is huge, not to mention everything attached to it. the radiator, intercooler, and other cooling elements are huge, and tall for that matter, fitting that stack alone behind the grille/bumper/under the hood would be next to impossible. then the transmission behind the motor plus the transfer case. thats a long package. then youll have to do axles, because a tuned 6.4 would shred the 8:8 in the ranger. so youd probably be looking at a 1 ton sas as well..
so yeah.. 4bt. or i like the ecoboost idea as well. or the new coyote 5.0.
i know all you og's already know this, jsut trying to put it into perspective for the op
07 Sport, 4.0, 4x4, leveled, 265 Duratracs, exhaust, intake, tune, stereo
08 6.4 F-350, smoked mirror and cab lights, H&S, 5" Flo-Pro, No Limit/XDP EGR delete