Yes, I described my experience with SE driver, and forgot that diff. driver is totally different beast. About half of the input voltage we`ll "see" at the cathodes, you are right. (The last time I tinkering with "long-tail" circuit it was on the guitar amp about 5 years ago:-)).
I forgot one other thing - diff. pair cancels even and sums odd harmonics, probably not a good thing with already "difficult" 6S45Pi?
Well, YOU must try it in the real world, and see/hear
I used rel. low values of grid-leak resistor, too.
But, with 2Vrms input on the grid of V1 we can expect about 40Vrms per anode out, too low for 300B.