Nice car! I've never actually owned a Mopar with a Dana (all 8-3/4, 9-1/4, or 8-1/4). The Dana, as you know, is a BEAST of a rear end. As far as I know, it was not ever actually factory-installed in an A-body (the biggest rear-end any A-body ever got, including the '68/69 big-block Darts and Barracudas was the 8.75.) *Maybe* Mr. Norm's Grand Spaulding Dodge installed it in some of his GSS cars? I'm also not an A-body guy, I've always had B-bodies and C-bodies. The 4-speed big-block and Hemi B-bodies (Charger, Coronet, Satellite, Roadrunner, SuperBee) are the ones that definitely got a factory Dana. The whole reason for the Dana wasn't to handle the power, per se. It was used to survive the shock loading of a very hard dragstrip launch of a very powerful engine in a heavy car with sticky tires without breaking a gear on the launch. Its massive overkill for cruising or even towing where the load is sustained but not impulsive.
That said, its not at all picky about gear oil, especially for your intended use. I would use a good synthetic with limited-slip additive. If you want to spend for Redline, then its great stuff. But honestly Mobil 1 gear oil works great in those old battlship-sized axles. I'd probably stick with something like a 75w90, but its not going to have any trouble on a 75w140 either.