I have welded 1/4" with that welder, the thing to do is grind deep V's into the joint surface like __/\__ and fill it in.
I WOULD NOT just surface weld a 1/4" plate with most MIG welders without this prep, it will be a weak link for sure. please remember that welder has a 10% duty cycle, that means short welds, 4-6" long single pass so you can.. filling in that 1/4" your probably talking 2-3" max at a time, and you need to stop and let the coil cool down.
this is true for almost any welder under $500, the times may vary but the idea is the same. if you want to weld more in less time, weld short welds (like one quarter of a small pipe, then switch to the other side of the frame, not the pipe), this will also help your frame not twist. the shorter the welds the less heat the coil can create before it starts to cool, so things like tacking can be done almost as fast as you can work. this is a "diminishing returns" concept though as you can still over work the box if you get careless.
if you want better duty cycle cheapish for things that thick, look at an 200+ amp ARC/stick welder, these are pretty much the way it was done at home until 5-10 years ago. I find these things in garage sales and flea markets for 100-150 all the time.
http://www.lowes.com/lowes/lkn?action=productDetail&productId=98103-1703-K1170&lpage=none
good luck. hope this helps