note that amateur VHF power amplifiers are intended to work with proper 50 Ω load.
Since you're planning to use it for plasma, if your load is not 50 Ω it can damage such power amplifier or it just will not work due to triggering high VSWR protection.
Also note that experimenting with plasma at VHF frequency can produce a lot of unwanted emission at air band and military band, so you can be punished by government for for interfering communications for aircrafts and military.
300 Watt is high power for VHF and frequency is very close to important bands like aircraft, so if you will use it for experiments, very soon you can expect guests, some kind of SWAT team with helicopters, etc...
At that frequencies plasma ball can work as a good enough antenna, so if you're still want to experiment with it, in order to avoid issues with government and military services, I suggest you to do it for at least within good Faraday cage and within underground basement to reduce emission. But note, that it still can lead to issues with government and military services due to unwanted interferences.