If I were doing this, I wouldn't mount the pickups to the pickguard until everything was soldered.
Just lay the pickups in their respective routes; run the wires through to the control cavity to be soldered to the pots; use the pickguard to make a small template of the pot/switch holes out of cardboard. Use the cardboard to hold the pots while you're soldering. This way you're not wrestling the a giant cumbersome piece of plastic.
Once everything is soldered, pop the pots/switch out of the cardboard and into the pickguard, mount the pickups. Done.
edited to add:
Whilst you're soldering, lay an old towel or something similar over the body to protect the finish from any molten solder.
You don't want to melt little craters into the brand new paint that you've laboured so hard to get right.
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