'78 is actually a transition year. Early production 78's should have the highly coveted fine spine 4.11's while late 78 trucks most came with FS 3.70's The 40 difs however came with the large pattern rectangular pinion flange while the FJ-62's have the better round flange than can be re-drilled for the small pattern.
Agreed on the flange pattern, however my understanding (based on both reading and first hand observations) is that USA spec 40s and 55s were 4.11 through 12/1978. Normally Toyota "cut in" new model years for the USA market in Sept of the previous year, but 1978/1979 was an anomaly for Cruisers in that the cut-in was January 1979. Lots of changes, body, underneath fuel tank, center console, squared bezel, etc.
As most of us know, there are many exceptions to cut-in on various changes, and wouldn't be surprised if there were a few 3.70s that ended up on late 78s; it was kind of like when they started using a newer version of something, they found a stash of old somethings in a corner of the Araco plant and used them up, even after they had cut in a newer version. Early 1960s were notorious for this (easy example is centered diffs on some early 40s but not others, etc.), but the practice continued through the 1970s. Another example is non-USA stuff - drums fronts continued to the rest of the world through 1985 (yes they still produced and exported FJ40s in 1985!).
Even with the diff installed, it's not too hard to count turns to figure out ratio. Just put the axle on jackstands, turn the flange and see where the wheels line up after 4 turns. And with the diff out it's even easier, just count teeth. Always 37 on the ring gear, 9 on the pinion for 4.11s and 10 for 3.70s. Happy cruisin'!
