What?
The only things you do with the lowers is cut it out a lil bit so it doesnt hit the frame when you lay the truck out... and weld the lower mount on...
And c channel?? I don't know what you mean.. Is this the first time you've bagged a truck?? Mazda's probably aren't a good first try, haha.. Here's a few pics of mine... I cut into the frame to set the bag in closer to the pivot point on the frame... You can see the bag mount welded to the lower and the upper mount...