The 250K pot following the coupling cap of the input triode is your volume control, I'm assuming it's wired this way to give reasonable sweep on the pot, rather than have a 1MA pot that you can't turn past a quarter. If you just used a 250K pot on its own, you would have no series resistance in the circuit & feedback inducing high end would possibly cut your usable volume before feedback. Alternativey, I'd just use a 1MA pot and tack a second resistor accross the ungrounded terminals to tune the pot sweep. You could experiment with caps to ground from the top of the volume pot as a high cut...say start with 0.0033uf or 0.0047uf, these could also be put on a switch?
Input impedances tend to be more critical in larger stage amps, smaller & SE amps an be quiote aggressive sounding anyway, vaue as draw may be fine for a softer tone but you could gou up to 5 or 10-meg. I'd also experiment with larger cathode resistor values at the input triode, you could switch to a couple of different values, look at 2.2K to be the max. Whether it's an Xtal or not itmakes little difference, only the new Shure 520DX doesn't like big input loads (1Meg+).
.1uf are fine for coupling cap values, as a starting point.
OT, I'd just use half the primary winding (3.8K, CT and one end).