That rectangular tubing will be far stronger than the C-channel it's sitting on. We used 3 X 4 X 3/16 to build dirt track race car frames, and didn't have any problems, so using it to support your oven won't stress it at all. Just put 45 degree gusset plates at the corners to minimize any flex and you should be good to go.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!