Where are you reading that? In my experience modern SKS guards come with spacers which should be enough to get you clearance for discs, but you never know I guess.
I remember fitting them to my gfs hybrid and involved quite a lot of bending/swearing which I'm sure ended up weakening the fittings (most have snapped). I did not consider spacing them out as I'm a dunderhead, that seems like the best solution . Tyres are 45mm so hoping 53mm should be enough.
Where are you reading that? In my experience modern SKS guards come with spacers which should be enough to get you clearance for discs, but you never know I guess.