This is really cute! I love the KAT profile and another white keycap set would be great.
In your 40s kit there's 2x 1.75u backspace, one of them should be a 1.5u backspace.
Also for the 40s kit it's generally best to keep all the 40s bars in one place. That being 2x1u, 2x1.75u, 2x2u, 2.25u, 2.75u. 2x1.5u is appreciated if there's room. 2x3u can be in either 40s or space bars.
Keeping a complete set of 40s bars in 40s makes sense since some of them (like 2x1.75u) is pretty much not used by any other keyboards and also that way 40s users don't have to pick up an extra kit.
Though even going with your current idea of buying 40s + spacebars (which I'd advice against) I can't find that the set has 2.25u or 2.75u at all.
If you want advice/help for 40s kitting the keycap designer discord has a 40s channel filled with helpful, knowledgeable people.