Best next upgrade?

That might be early enough to still have the odd tapered axle setup with the threaded flanges or whatever? I think the years you want are 1965-67, someone correct me if I'm wrong.
Looks like you’re right that a 63 axle would be tapered/keyed. I’ll keep looking
https://www.hemmings.com/stories/article/mopar-8-3-4-inch-rear-ends