1) From the A pillar back, the trucks are identical
89-92 have bigger wheel arches. 93-12 trucks are even more similar, the beds are a direct interchange.
2) I could put an 83 front end on my 1990, but I have to change the grill, fenders *and* hood (and maybe the rad support)
Yup. Could put a 2012 front end on a 1993 too.
3) Both trucks use the twin I-Beam front suspension
So do the 93-97's.
4) Engines and transmissions are different, yes, although there is overlap (2.3 and 2.9 were available in both)
Yup.
5) While the dash and door panels are different, I have seen my seat in a Gen1
Most smaller Ford vehicle seats fit too. I have had BII and Explorer seats in mine. Mustang seats are also a popular swap.
I doubt they changed the base bench seat any more than they had to.
6) My rims were available on a gen1, and I think the standard steel wheels with center cap and ring on the gen1 were also standard on gen2 with my rims being optional
They were, the second gen had alloys that were also available on the 93-97's.
7) Ok, I do know that the gen2 is wider, but since I've been told that the hood and grill (and bumpers) will interchange, I'm not sure where the extra width is
93-97 has more flair in the body for wider axles. Nothing sheetmetalwise interchanges or fits on a older truck. Beds do bolt on but look very out of place. Bumpers might too but they would look weird.
8) "big" mirrors (the fold away ones that stick out from the body) are the same mirror on both trucks, are they not?
Fullsize trucks had them too.
9) I can't speak to brake parts, rear suspension, etc.
My '85 has rear springs from a low mile '08 Supercab. They played with rear brakes but I opted for an Explorer axle so I bypassed all that stuff and never dealt with it.
IIRC the front calipers stayed the same until 1995 when they went to the bolt on two piston calipers.
10) Aren't the driveshafts the same?
83-88 had a myriad of transmission options and they were just about all different lengths. They were starting to get more standardized by the time the 89-92's came out and narrowed down to just the A4LD and M5ODR1 which are both the same length. About the time the 4.0 came out they also started going with much bigger u-joints too.
I am restoring my '85. When I see a 89-92 in the JY there is usually little reason to rejoice because they have about as much in common with mine as a Toyota. None of the sheetmetal does me any good. Don't care about the powertrain (4.0 trucks did have better axles than 83-88 but I have better axles than a 90-92 4.0 Ranger), interior is totally different, electrical is totally different.
Tail lights, tail gate (although they usually have the big aluminum plate that is a DQ for me) seat, and if I go on a wild goose chase sometimes I might be able to find an electrical connector I need. It might be somewhere on the truck but it typically won't be doing the same thing it is doing on my '85. That's about it for interchangeability in my world.
I am converting my truck to the 91-92 4.0 style A/C because the hose routing will fit my 5.0 better though.