I think 280V might be a little low for the screens, 300V would be better with the plates at 350V. That will leave 50V to drop yet, 10-12k would then work.
Maybe we ought target 5E3 voltages of 370-320-250. 5B3 voltages do seem kind of low. @Jerry garrcia you are looking for a more clean sounding amp right?
I’m going to keep giving my own sketches on the layout just so you have options. @printer2 has come up with some good ideas that I will incorporate in mine. My goals again are to get an amp the works using the constraints of a board that’s already built and installed in the chassis.