coupling a 3phase alternator to a Diesel car engine

The xud and dw8 were used in a lot of cars, and your right the dw8 also went in the ldv pilot, with a rwd 'box.

If the o/p is using a induction motor as a gen regulating the field coils isnt an option, the field coils would be the rotor, to get the rated voltage you'd need to run the motor a bit faster than rated rpm, about 1560 for a 4 pole motorm which automatically messes up the frequency by a few hz, so its a bit of a juggle between correct freq and voltage.

Some of the older dyno test beds were sold with throttle actuators made out of Ford Escort windscreen wiper mechanisms with a pot connected for feedback

These days, a half decent servo would more than suffice for mechanical throttle. Personally I'd magnetically couple it somehow so if you did have a malfunction, it would release and kill the throttle.