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I have the same doubts. Even if the 1.4 HDI's gearbox is still a "MA" type like the AX, I believe that there have been some modifications since. I have only seen the MA/5O gearbox on pics and from what I could see the body is not exactly the same so I'm not sure if the holes for the screws are exactly in the same position. The best is to see the gearbox before you buy it and check.
Secondly, that gearing will be way too high for that low torque engine (75 Nm at 3700 rpm). I have 44,7 km/h/1000 rpm but with an engine that has 25% more torque than yours (97 Nm at 2250 rpm) and at lower rpm which means that it has around 40% more power at 2000 rpm. And it is already quite borderline, it's fine for highway cruising but I have to mash (80%) the throttle on 3% inclines to keep 90 km/h. I increased ratio by 19% and top speed dropped to 140 km/h. What you want to do is +40%, I think the 5th gear will be undriveable with top speed no more than 110/120 km/h on flat and without wind.
Try the diesel gearbox (after 92), it has 20% higher ratio (43x33 and 17x61), that should be enough.
As for the tires, I mounted them only on front because :
- it's a front wheel drive so you only need them on the front to have a bigger gearing, I have no driving issues with having different sizes on front and rear, it's just 5,7%.
- they don't fit at the rear anyway (I have only 2,5 cm free with stock tires and the 155/80 would probably rub on the body)
You don't need to put bigger tires. You have enough room for improvement on the gearbox ratio to keep the original size. It's always better to keep the lowest tire size, less rotational weight, and change the gearbox. I have some good experience with the Yokohama Bluearth which exists in 145/70.
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