Speaking from Experience, replacing the 4sp with a 5sp in a 1.5 is not going to get you much except a lot more shifting on the slightest incline. My 82 was great to drive with a 4sp but struggled above 100kph. I currently own a 79 in the states that had a 1.5 with factory 5sp, was OK but not ideal. On the hwy I would have to shift quite a bit on the slightest incline or headwind much better with the fresher 1.6 and even better with the 1.6 with turbo installed. The same sections of road with the 1.6 I no longer had to shift to just maintain speed. The 1.5 is a higher RPM engine than the 1.6 also