There are a couple wiring differences in the 73. Not a big deal just a difference in the connectors so it takes some work when replacing stuff.
The 73 seats IMHO are the most comfortable, I loved em. Loved the full vert for summertime cruising. I really like the grilles from the 73 too.
I like the power everything on my 89, I like the 700/241.
There are some differences in the floor between the 73-75 vs all the others.
I didn't like the old style glass fuses in the 73.
I liked having the old school door panels and dash in my 73, but I also like my newer ones in my 89.
I had collected everything to make my 89 look like a 73, then the 89s front clip would go to the 86 dually. But I have decided to just go full vert on the 89 and the 73 front clip is going on my dually.
Really boils down to I like the 89 alot better, only reason I would buy another 73-75 is for the full vert.
I also like fuel injection although my 89 Jimmy is a diesel