I agree with pugsy, do not make any relief cuts or do any work anywhere except on the weld itself. If you are quick enough to planish the weld while it is still hot it will reduce the chance of cracking, but that is history now that its already welded. Do not reheat.
I prefer gas or tig but in the future its important to make sure the patch fits perfect before welding, and with a mig just do a spot here and there and let them cool naturally.
When planishing cold, grind the weld bead almost but not all off the front and back, then put a dolly with a similar contour on the back side and hold it with a good amount of pressure to the weld and strike on the dolly with firm blows up and down the weld area, you should hear the ring of the hammer hitting the dolly. You will see when it starts getting close to the right contour, then start using a straight edge, and keep raising the lowest areas. Its not that difficult, you can do it, good luck.