Quote115VAC, 60Hz primary = what it needs from the wall (mains) to go.
correct
Quote300-0-300 secondary = one tap will be +300VDC, another -300VDC, and the center is neutral. 600VDC across the entire secondary.
The voltage is still AC at this point. If you measure across the 2 legs with a meter set to AC you'll measure 300V.
I used a 270 in a tube amp that I built. Most SS amps use transformers with lower voltage and higher amperage.
Do you need to replace a transformer or are you just curious about the ratings?