Nothing, but...
Both front and rear sight holes have the same spacing, so no problem there.
However, that XS front sight model is a single piece; there will be no adjusting for windage, and the only elevation adjustment you can make is to file it lower, which will raise the POI. XS Sights does make a regular (non-winged) sight set for the 336 with the same kind of 1-piece front sight, so they do make a 336-specific 1-piece front sight. Elevation will be somewhat off because of the 336Y's shorter barrel, but the rear sight may have sufficient adjustment to accommodate without getting too close to the limit.
Worst case, you could buy the winged rear, install it and shoot it to figure out what front sight height you need, then order the correct height white stripe front blade and install it into the existing ramp.
I would call XS Sights and get their advice.
The other option would be to get a set of Skinner Sights. I'm very partial to the Alaskan on Marlins (including the 336Y), but he does make both front and rear winged sights if you insist on that type. An Alaskan rear paired with an XS white stripe front is ideal, IMO. As soon as I've settled on a 190gr cast load for my 336Y, I'll figure out what height front sight I need and order the appropriate XS white stripe from Brownells.