im a electrician for 23 years and weld alot also, you cant get 3 phase from a alternator, 3 phase is 3 seperate power sources of the the same voltage, from seperate power sources, also where I work at a chemical plant, he have alot of dc powered systems, and we run dc rectifiers all over the place due to we have power failures, and have back up battery power banks, you can run a dc rectifier that converts dc to ac voltage , its a common practice, also i recommended going to 1:1 ratio rectifier due to your voltage/amperage range,they make a varied range of size conversions, you can get them that would convert 12 volt dc to 240 ac, but you amperage would be less then half , its higher voltage-less amperage or lower voltage-higher amps, its just like a power convertor that you can buy to plug into your cigarette light that you can run ac devices like a laptop, small tv or whatever, but off the alternator on a different scale, hope this helps.
its still a cool set up you have.