I actually started working on a quest a while back because I got fed up hacking the Zelda ROM. The way Zelda does its overworld map is very frustrating, instead of being a normal tilemap they're stored as columns, there are a limited number of columns and they're really only designed for making Zelda's overworld. I've had no luck hacking the columns either, I don't know if I was doing something wrong or what but changing a tile in one column would mess it up in other columns as well, but not all of them, almost like their data overlapped or something. The dungeons are similar, but the biggest problem with them is a lack of a decent dungeon editor, ZeldaTech's dungeon editor never came out and the only one I know of causes more problems than anything else. Of course it could all be done in a hex editor, as Zelda Challenge was, but I need something visual when I'm making maps.
Overall, once you learn how ZQuest works, Zelda Classic is just easier and a lot more flexible. I used
this tutorial when I was learning, it's really thorough but kind of out of date. The information should still be relevant, some things may have moved or been renamed though, they cleaned the interface up a lot in the newer versions.