If adding spacers and not sprockets, then add to the non-drive side and keep the dishing as even as possible, you need the strongest wheels possible. Those disc adaptors look flimsier than tin foil, probably not needed anyway but would depend on total loaded weight and where you plan on riding - flat or hilly?
And would be interested to hear more about those rims, especially if they're available in 26". I have D521s for my Yates but it's always good to know what other options are available