a quick RPM to MPH calculation says says a engine RPM of 4000 (I would assume the upper limit of these flat heads) 3.91 final drive(1-1 transmission final drive and 3.91 rear end gearing), and a 28 in tall tire would net a top speed of 85.2 miles per hour. A taller tire would increase this, a lower final drive(bigger numerically) would decrease it.
dodgeb4ya's
example above tire 8.20x15 is 29.56 tall, 3.36 final drive, 80 mph engine would have been 3057 RPM. I would assume a very reasonable RPM for extended time.