I have a four speed I6, 258. The other day I was doing 70-75 going down hill with a tailwind. It was fine, but not safe. I avoid all highways and if I have to ill do the frontage road or the far right lanes on the highways. CJs aren't sports cars and shouldn't be, they are very squirmy due to bump-skid steering.
'86 CJ7, I6 258 four speed manual. See ya on the trails!