I've found that each of the various Minors I've owned over the years has had a slightly different speed at which they're "happy" cruising. When we took our 948 car abroad it cruised comfortably for an hour or two at a time on the motorways at 60-65mph. The 1098 car has a slightly higher top speed but various vibrations mean that the car doesn't feel comfortable maintaining speeds of much more than that for long periods.
In all honesty, whatever engine and gearbox you fit, the Minor produces quite a bit of wind noise etc so will never make a great motorway car!
As someone else has already found, the Minor speedo is not the most accurate instrument in many cases and tales of cars which will go "round the fuel gauge" are probably largely the result of an optimistic speedometer