My '59 Viking 40 will run at 55 with no problem, and I've had it up to 60-65 mph several times. Although it is really rapping out that little six banger. It has a 292 with (I believe) the stock 4 speed transmission and single speed rear end. The tires are 8-19.5 which I think are little small, it would probably do better with 8.25x20's on it.
I think the 292 makes the big difference.
John