You are shrinking whether you know it or not...welding always shrinks...no way around it. Punching little holes (plugs) from some scrap is a great way to go. You can hammer and dolly the plug to make a perfect press fit in the hole. I usually end up punching a plug that is slightly too small and then setting it on a steel surface...hit with a hammer until it is stretched to fit the existing hole for a press fit. Gas weld or tig with no filler rod, stretch the hole area only to get the metal back perfect...a lot of work, but worth the effort...or mig-weld, grind and fill...another method if someone is likely to get the car after you and want to change it back...simply fill the holes...not a great technique but a future owner might appreciate it.