Internal Wire Gauge?

18 gauge for the power wiring to and from the mains xfmr and 20 gauge for the signal wiring to and from the output transformer. The wiring should be UL1015 listed (600 volts and 13 amps for #20 and 18 amps for #18 at 40 degrees C)

You should take these questions over to DIYaudio where you'll run into a lot of builders. :)

Best,

Erik

2.0 welding cable should be fine. Make sure it's grounded well.
Pure silver multi strand, I figure about 25k for cable, 500.00 valve amp..
LOL

Beat me to it! :-)

The wiring should be UL1015 listed

That's really specific and unnecessary. UL1015 is 600V  PVC shielded appliance wiring material. It's in an enclosure, there is often no need for shielding at all, and it would be detrimental to the build in many locations.  Where shielding is required, there are any number of UL style wires that would be suitable depending on where used, how supported, etc.

I'm not saying you should not use premium wiring, however, for price consideration:

If/when you take the bottoms off vintage equipment or backs off vintage speakers, you will be amazed how small and common the wires were/are.

Keep in mind, this is what made those vintage audio companies famous.