Or, you could obtain an original accessory
wiring junction bock (original for a car).
If wired as per instructions, you'd have four additional "always hot" circuits and four additional ignition ("switched") circuits. You would then want to consider using in-line fuses?
I have bought a few of those switches NOS for about $10 (a few with free shipping). The 53/54 versions have heavy-gauge plastic-coated wiring - I suppose the earlier versions have braided wiring. Better installation instructions should come with the switch (if truly NOS).
I am being only partially serious about this. If I was converting to 12v, I'd consider a different option, probably a more modern junction block with fuses in the block.