[mongocanfly]

i really wouldnt trust a 2x4 for a straight edge...you could drill a hole in the front of the panel opposite the dent and use a round ended punch thru the hole to knock the dent back out...just lay "face down" on a hard surface..working slowly and moving around the dent you could get most of it knocked back out..weld hole back up when done...for the bow thats in it, I have layed 2x4s on top of bowed panels and just stood on it and bounced a little..a little goes along way though..
I've used a dent riser to remove dents in gun barrels before...very similar to what mpc described...2 wedge shaped pieces of round stock pulled against each other with allthread...works great
mpc is a master metal man so what he says is all good..