[dsraven]

yup, shrinkage from welding is my guess too. like Bam says, hammer on dolly ONLY the haz to expand that area. I use a piece of sandpaper on a block of wood to scuff over the area after making it all colored with a big fatty marker. the sandpaper will show up the high and low spots where a little more work is needed. after that I use a shrinking disc to heat up only the high spots and then a cool wet rag quickly to cool off those heated up spots to shrink then like the metal around them. you could try a laundry iron for that if you get brave. it is only for the last minute final stuff so you don't have to use a bunch of filler.
if doing something like this again, use tack welds and let cool between them. tack, hammer on dolly that tack when still hot, cool, next tack, hammer on dolly the tack, cool, keep going till there is no space between the tacks. space the tacks out to hold the piece in shape so the gap stays consistent along the whole edge. like do one at each end, one in the middle etc, before doing a bunch like in the middle or something. don't start in the middle and work your way out unless you are carefull to keep an eye on the overall shape and gap spacing. otherwise you will find the piece has slowly found it's way into the scrap pile because it is outta shape. always let it cool off till you would be comfortable sticking your tongue on it before tacking/welding again (thanks martinsr for that tip, lol). having two projects to do at the same time is good so you aren't tempted to go too fast.
since the project is supposed to be flat across from side to side, you could hammer on dolly from the back side using something fat like a steel workbench top or a piece of flat steel as the dolly. be aware of what type of crown your body hammer hammer has as well. a little research will help figure that out, or trial and error. if you have a dolly that is shaped like the curve you can use it as a hammer/slapper.