If you still have 13 inch wheels 50 is right near the engine's sweet spot.
The 144 makes peak torque at 2,000ish RPM in stock condition. That's where a car will feel the best at cruise.
With 185/80R13 tires (about 24.6" diameter)
a 3.50:1 axle ratio
at 50 MPH
your cruise RPM will be about: 2,454 RPM, past the peak but strong enough there to maintian that speed.
At 70 you would be turning 3,436 RPM, still coming close to that peak HP number at 4,200 RPM, but well out of the engines comfort zone.
Remember, too, that the 144 and the 170 only have 4 main journals holding that long crank and were never meant to be a high-RPM engine. | 
