About 6:1 would be nice to try with first, maybe you can go down to 5:1 if it feels like it has enough power for it.
Yes, you can build a diff, but it takes a lot of work, machining, spinning with lathe, milling and lots of other operations. It would be easier to get a Baja-diff and gear it down 3 or 4:1 with pinion and diff-gear, and take the rest by gearing it down 3 or 2:1 by using the bevel gear to connect to the clutch with an axle.