On 4.2 it doesn't matter what IOS30 you install, it cannot brick you. 4.2 accesses only IOS70 to run the system menu. Install IOS30 v1040 as recommended, but if when you get to Section  and run your SysCheck again it stalls on IOS31, you should follow the directions Krank quoted above from the FAQ.
If any of your IOS give you a download error when downloading through DOP-Mii you should first try again, it might just be a random error. If it still won't work follow the offline installation section's instructions.