Honestly, I don't remember exactly... 275/60s maybe. They're something like 28" tall. I've been watching for a deal on a open 8-3/4 3.08 to pop up. I'll swap it out eventually. Definitely don't need a locker in Hoopty.
A challenger runs a 28.6" tall tire so not far off.
Straight ratios, the 8HP/3.06 combo runs a 14.13 overall first gear ratio. If you factor for the tire size difference, Hoopty is like running a 17.08 overall first gear in a Challenger.
Comparing Hoopty to Butters with a 26" tire and a 3.21/3.23 ratio would feel the same.
So a 2.71 or similar would feel like a stock Challenger and a 3.21/3.23 would be too much. So there's your range. :D
Looks like a 3.08 would be a nice combo in Hoopty.