In my experience, getting a 50 amp alternator from a similar year Datsun is a direct bolt on, the external voltage regulator does not even need to be changed. The positive and negative wires should be changed to something thicker to handle the extra current.
You might have to get a new belt, to keep the laternator closer to the engine, and away from the steering idler arm.
There is a black wire that goes from the alternator case to one of the screws that holds the voltage regulator to the inner fender. This wire is very important, it grounds the cab to the engine, and the battery. If this wire is missing, the cab will try to ground through the throttle cable and destroy it and possibly leaving you with a throttle stuck open. There is also a black wire from the alternator case to the negative battery cable.
This is the alternator on a L-18 engine in one of my project 521 trucks. This alternator came out of a Nissan Pathfinder, Pathfinders are real common in junkyards in the USA. I do not know what Nissan calls Pathfinders outside of the USA.
The L-16 engine in a 521 originally came with a stamped steel lower mount, held with M8-1.25 bolts, If you have the stamped steel mount, your alternator fit may be different.
This shows the bottom alternator mount. The mount is the same width as the space between the mounting ears of the alternator, and more importantly holds the alternator in the correct forward and back position for the V-belt grooves to line up properly.
This is the top mount for the alternator. The slot for the top mount actually matches the radius the top mount bolt moves in.
A better view of the top mount.
This shows the view looking down on the belt, in pretty good alignment.