If you think the spinboxes in the underground levels are too yellow, you can put passive blue blocks in front of them.
Also, since the blue blocks and item boxes are about the same size, you can use the blue blocks for the ground as a sort of scale for the actual level. Then you can put the green ground on top of that, but don't delete the blocks underneath.
All the maps for Mario 1, 2, and 3, as well as maps for Super Mario Bros. 2: The Lost Levels, can be found here
. Just scroll down a little, and you'll find them.
Good luck with your levels!