Squareback vs round back alternator?

Hello from Berlin, Germany! My name is Christian and I'm a mechanic usually working on European cars.
Just a helping a friend with repairs on his Plymouth Valiant V-200, L6, 170cid from 1960.
We're facing some issues with the alternator / regulator and we hope you can help us out. We're thinking about replacing alternator and regulator, preferably an electronic one if possible. Could you help us out which would be the right parts? We guess alternator and regulator don't match.

The voltage is fluctuating by checking at the Batterie +. Voltage went up till 18 Volts, so I checked the regulator air gap btw the armature and the magnet core at the hanger side which was found within specs and I bent the spring tension up because the voltage was found to be to high. Now its withing 13 and about 15 Volts but fluctuating very much, a bit less with all consumers on.

Regards, Christian.

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