Both fit just fine. The 60/40 is just harder to install because of the transmission, and making another set of 1980s seat tracks bolt down in the center of the cab.
I've only seen it one time in my 52 year life, but the driven gear inside and on the tailshaft has a small metal clip. The clip had broken on this truck that kept that inside gear locked on the transmission shaft. Then the gear was push back down the shaft so the drive and driven gear didn't...
I was going to ask what year we are dealing with. Sounds like a 1987-91 now.
I had a 1983 truck with a worn out 305cid. The CEL would come on when she ran 2 quarts (or more) low on oil. That is that 2nd oil pressure sensor, down by the oil filter.
Just more data for someone else....