Yep, a 245 tire has a 7.5 - 9.0 recommended rim width, so it's also a good fit for an 8" wide wheel...
I know LOTS of people run 275's on an 8" rim, mainly to avoid buying new rims. Just because you can/did, doesn't mean you should... After all, you said it best "Theres a reason late model Mustangs come stock with 245s on a 8" rim" - they would've put 275's on if Ford and the tire maker THOUGHT they could. A 275/40-17 carries a rim spec of 9.0-11.0, but everybody is smarter than the tire makers...