If you have any sort of handheld rotary tool or dremel then you don’t reeeeally need to remove the c-channel to cut it. As long as it’s secure, it’s more or less safe.
Only do this as a last resort. I’ve found that dremel cuts are really hard to do well, and all the vibrations loosens up essential structural components over time. I’ve had to retighten really hard to reach screws after prolonged use of a dremel. At a competition, this is practically the only option though.
That’s fair enough as a general rule of thumb, but I wouldn’t make any changes to the chassis once its already squared. For something like that I’d dremel then re-tighten anything around it.