The LT1 can actually burn 87 octane all day. My OL drives a '96 TA and it has knock sensors. I've never heard 1 ping on 87 octane come out of that thing. Her Opti has 100k miles on it too, it's fine. I dunno what kinda mileage she gets, probably close to 20mpg. I know it's way better than my Caddy and my Camaro though
I think people just jokingly shit talk the LT1. It's actually not that bad, especially for a DD. It's so underpowered that it's pretty hard to hurt it too.
You're better off delaying that Mustang project indefinitely. Fuck Ford!