Do you mean do this in addition to how it is shows it wired or instead of how it shows it wired?
RE SPDT: NOT in addition to (sort of instead of).
Depending on where your switch is placed in the enclosure relative to the pots and board, it might make more sense to wire the centre-lug of the SPDT directly to the VolPot's lug 3 — it's what I'd do if it meant a shorter wire (equalling less spaghetti and less antennae to attract radio interference).
Maybe you're still thinking too physically. What I mean is ...
I still have trouble getting past the physical elements and placements — electricity doesn't really care how three components are all physically connected to the same point, an electrical node; so in this case whether you connect the SPDT middle lug to the board or the pot doesn't matter so long as it's the correct shared node.
Look at the lavender wire running from lug 3 to the board. Doesn't matter where along that you connect, you could interrupt that wire and stick the middle lug of the SPDT in there, then continue the run of the rest of the lavender wire from the middle lug to the board as show in the layout diagram. Physically, lug 3 is no longer connected directly to the board, but electrically nothing has changed (hence the "sort of instead of" comment at the top of this post). Splicing into the lavender wire could be the best way for you to physically arrange things in your build (or not).
For ex: If Mcknib were to add a SPT switch to his build, it would make sense to put the SPDT between pots. Mirroring his build for yours: Switch and pots together, SPDT middle lug direct to Vol 3, then run a wire from SPDT middle lug hidden under the board to its proper contact point and you've got a cleaner looking build.
___
|___
|_____
._|
Is there any reason you're using that specific 3PDT connection method? To ground the input of the effect during bypass?
It is a bit confusing, until you break it down. Easiest way I've found to get my head around 3PDT layouts is to omit the pole with the LED — just look at the in/out connections, then add the LED aspect back in. There are other 3PDT layouts that still ground the effect in bypass, but have the jacks wired to common-lugs.
