If you fire up the preamp with a rectifier tube and no signal tubes, the B+ will go pretty high (250-300V) when those devices are not drawing things down. The rectifier will probably bring up the B+ first anyway, hence the recommendation for 250V parts.
I'm thinking about the umbilical cables and connections. I was planning on two separate umbilicals - one with the heater supply and one with B+ (unless you think I could run both together?). I was planning on twisting 3 x14 gauge stranded PTFE insulated wires together for each umbilical and then passing that through some insulated sleeving. Finally, I'm going to cover the whole thing in copper braid which I'll ground at both ends to create a Faraday cage. The umbilicals will be about 5 feet long. I know I don't need that large a gauge for the current but I have it already. The third wire is a spare or for audio ground if the braid isn't working properly.
I think your plan for the two pairs of umbilical cables will be fine. That's what I use but each pair of wires is just loosely twisted around each other; there's no shield. I live in a rural area with pretty clean AC lines so the unshielded power supply wires might not work as well in another environment. The shield certainly won't hurt and it may prevent some noise pickup.
With the umbilical plan approved i'd like to turn to the CCS - specifically, how to mount this so the heatsink protrudes through the top plate or at least ends up mounted in a way that the PC board is not baked in the heat flowing upward from the two mosfets strapped to the heat sink. The instructions are to mount all components on the printed side but that places the resistors and caps upside down and inaccessible. If I mount it so the PC board is oriented correctly (like in my picture) then when I flip the top-plate over, the parts seem like they will be cooked. There are ventilation holes in the PC board. Suggestions?
My K&K boards are mounted as you described, basically upside down when the preamp is in use. I don’t have any ventilation holes in the top plate, and I haven’t run into any problems. By the way those heat sinks look huge. How tall are they? Maybe K&K is now stocking a larger heat sink than when I bought mine.
What I have done in that situation is to cut a rectangular hole in the chassis to allow the heatsink to poke through, then mount the board on #4 standoffs to the chassis plate. You should not need access to the parts on the board provided you set them properly before installation. If the heatsink is partially in free air and you aren't demanding too much from the heatsink, you don't need to be concerned about the resistor and trim pot getting hot.