I’m preparing to drop in a Starion 75-amp alternator in place of the 50-amp unit in my ’87 4-cyl. However, I’ve been looking at the charging wire running from the alternator, and it’s a pitiful thing; I haven’t measured the conductor to be sure, but it looks like no more than a 10-gauge wire.

Looking at wiring charts, it tells me that a 5-foot run (which is about what it is from the alt to the battery) at 75-amps requires a 4-gauge wire. That’s a big difference from what’s in there!

Should I upgrade that stock wire before putting the new alternator in?

What little I've learned from restoring motorcycles and working on cars for 30 years is, manufacturers do not put more than what is needed in their electrical systems, because on a production scale adding $0.10 to the cost of a single wire gets very expensive when they build thousands of them in a month.
In other words, the original wire was probably barely sufficient for the OEM 50-amp current, in addition to being 30 years old and having decayed in that time, you should upgrade it.

I've decided to replace that wire with a 4-ga wire and a slow-blow ANL fuse. That should cover everything.