I'm wanting to replace the factory buckhorn sights on a 336Y I just bought with a particular set of Winged XS Ghost Ring sights, problem is it's listed as specifically for an 1895. As far as I can tell, the 336 and 1895 share the same bolt pattern for their scope bases and maybe front sight; so what's stopping me from just buying these and installing them on a 336?

Thanks in advance

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.

Maybe you know this, but just to be clear for the OP. Windage and elevation are adjusted on the rear sight. There are two sets crews for adjusting windage, and you screw the aperture for elevation.
XS includes a plastic post you can superglue in position if you need to determine front site height.
I have XS on my 336 and 10/22. I always end up bottoming out my aperture. And still need a little taller front sight.

Since OP's new 336Y is a new-production Marlin, there is a chance it has some QC issues. While most of my Marlins (including the 336Y) have the Skinner Alaskan noted above, I have the non-winged version of the XS Sights front/rear set on one of my JM 1895 Guide Guns, and like it. However, the lack of any adjustability in the front sight can present a problem if his Marlin is one of those with slight misalignment of the barrel.
I prefer my sights to be reasonably centered; while the adjustment range of the XS rear is pretty wide, it will look crappy if it's significantly off-center. With a dovetailed front sight, minor misalignment can be more gracefully managed by moving both front and rear sights in opposite directions, rather than making all of the adjustment at one end or the other.
Additionally, as you noted, needing a taller front sight is common; this will be even more likely with the shorter 336Y barrel. While the test post comes with a new front sight blade, the 1-piece version OP posted doesn't, since you're removing and replacing the whole dovetailed ramp with a 1-piece unit. Buying a new (taller) white stripe blade to put in the Marlin ramp is a few bucks. Can't do that with the 1-piece front sight OP posted, hence my suggestion above of XS winged rear and white stripe front blade in Marlin ramp.

I have a set of those same sights if interested.  Took them off a 336XLR I picked up here a couple months ago.