There are lots of different ways you can do this, without the difficulty of slitting the pipe.
You can make a U shaped tab that fits over the blade, and weld the pipe to the front side of the tab. Like this: oU
Or, the last one I did, I used 3/16" flat, and attached the tab to the front of the blade, and then I welded the pipe to it. Like this: ol
Works just fine. And it only took a few minutes. I have 2 of these tabs, on a 6' pipe.
Aligning the tabs with the existing bolt holes on the plow, makes it an easy job.
I will post a photo, when I get some time in the next few days.