What grit sandpaper should I use on my drumsander?

You don't say what machine you have or the max height difference between your lowest and highest blocks. On my Supermax 19-38, I would start with 60 or 80 grit depending on that height differences. I prefer to glue things up with the minimum variance in height and then take as many light passes as needed to create an even surface.

You can then go up to as high of a grit as you wish from there. Just go with light passes on the high grits as I can see too much heat glazing your end-grain blocks.

If you're working with a height difference of 1/4" or more, I'd definitely start with some 36 grit and work up. The drum sander does all the work so you don't really need to worry about the sweat equity involved in removing the sanding marks from the 36 grit. The thickness drum sander really is fantastic tool. I could easily get lazy using it too often.