I regularly drove my GPW 20 miles one way over winding 3 lane back roads in NY in the summer. One Fourth of July I drove it to the fireworks at the capital, a 35 mile trip. 50-60 mph indicated. Two things made the trip one I wouldn't want to repeat daily. One, by the time I got home in the chilly evening, my rear end was almost numb. WWII cushions left a lot to be desired. Second, at that speed , the brakes were not very quick to slow down the jeep when someone pulled out in front of me. The jeep had all new shoes, new wheel cylinders, lines, hoses, and master a few months before.
A 3a will have far superior seats, and if you tuned up the brake adjustments to factory specs and you had the high friction pads, you might have better brakes for highway use. (There are brake rebuilders who will custom install new high friction linings, instead of relying on cheap omix-ada or Crown reman shoes.)
The other thing I would do before using a jeep for any highway running is instal turn signals with bright bulbs. Everyong thought I was waving at them using hand signals.