If I had to change my g\box,
Which would be better
A14 or standard a12 ?

If a14 g\box, does one have to do mods and do have to change prop shafts?

What will be the perfomance differences?

My car is a 74model 1200 Datsun.

Which is better? It partially depends on if your 74 1200 is a B110 or B210.

I think the B110 gearbox (ribbed case) is the better for an A12, as it is smaller and lighter. First gear gives more leverage as well.

On the other hand the B210 gearbox (smooth case) is stronger, but heavier. 1st gear is generally higher, meaning less acceleration but closer ratios.

But if your car is a B110, the B210 gearbox might have problems with the gearbox mount or reverse light. If so a little modification works. I'm fairly sure the same driveshaft is used.

In summary, I would use whichever gearbox I could get for less money, because both have pluses and minuses.