Well Advance Map is easily the best overworld mapping tool - in fact the whole Advance series (A-Trainer, A-Mart, A-Text etc) is awesome and would definitely be my first choice. I actually have no clue how to edit world maps, cos i haven't got around to it yet on my own hack, so i can't really help you there. I think A-Map has a tool for that, but i have tried it myself - I know one thing though, that it's really complicated.