Flying Pram, you're right about the rectifier pack, you would have to leave it in and tap off the stator windings for the AC output.
I've had a look at the theory and I reckon it could be done with the right kit.
If you could find an alternator with six salient pole pairs you would need to rotate it at 8000 rpm to get 400Hz, that's not outside the realms of normal operating speed for an automotive alternator. You could also expect an output of 70A which gives a power rating of nearly 2Kw which should be plenty.
You will also need a DC supply to energise the field. Maybe that's where you went wrong with regards to your low output current. Were you just running it with residual field excitation?
I made a homebuilt generator many moons ago in my uncle's workshop. It was made from an alternator off an old Volvo and a Suffolk Punch lawnmower. It worked a treat. My uncle ran a used car lot and he used it to jump start the old bangers with flat batteries. He also used to take it camping with him to charge up the battery in his caravan. Although this one was a 12vdc output.