By looking at that picture of the NisSport adapter, I would assume there would be no clearance issues with the block heater. They do not appear to occupy the same space. Take a look at
this picture to see what I mean.
You might want to drain the oil and temporarily uninstall the NisSport adapter to make installation of the heater a bit easier though. That might help. Then again, it might not be worth the effort. Depends what kind of tools you have I guess. =]
As for the Custom Steel piece, I can't remember what it looks like, or how it installs, so unless you have a pic of it installed too, I can't make any guesses about it.