I had same issue before and found solution from this post: http://www.minecraftforge.net/forum/index.php?topic=42150.msg223961#msg223961
If there are some class file contain NO IMPORT and 3+ BLANK LINES at the end of file, then the building will fail or just hang at 82% retromapReplacedMain.
You can find what class file the building is hanging at by log file:
(mod working folder)/build/taskLogs/retromapReplacedMain.log
In my experience, it happens in forge 1.10.2#?~2221, but forge 1.7.10 is working fine.