A good alternator with an AVR board should stay pretty steady on the voltage over a fairly wide rpm range especially if the prime mover is fitted with an electronic governor,A diesel gen set will inherently be more stable on the rpm any way, otherwise generate at a high voltage and use a motor generator set as they used in the old days, whatever you use it is not going to be that portable it will have to be truck mounted.
Out of interest how thick are the filaments in this tube, I have some tungsten rods for tig welding and the 3.2 mm ones will run at about 90 amps go much above 120 amps and they melt.