It's 126mm but VERY easily used with 130mm and STI stuff. Been there, done that, a few times. No frame work needed.
The Series 5 used a Hyperglide 7-sp cassette, per the catalog. These can be updated to 8/9/10 version with an axle and freehub swap.
Schwinn 1992 Paramount -- Front Cover
Welcome to the forum. Glad to have you here.
For ideas, there is a great thread here with exactly the kind of upgrades you mention.
I did a Series 2 and a Series 7, had no problems at all.