Problems patching a 1/4 panel

Ok guy,

I gotta real problem and I really could use some help.
I'm trying to do a patch repair on my 72 demon on the passenger front 1/4 panel. Today I fit the patch in. Lined it up everything was perfectly lined up.
I tacked it up with my mig set at power level 1 feed speed 2.5 gas at 10 psi and .03 wire.
When I check the patch the metal pulled in really badly so I now have a 1/4 inch indent.
What should I do. What did I do wrong.
I understand the metal will move a certain way when heated but who do u stop this from happening.
Should I just finish tacking it in and than fill it with fibreglass reinforsed bondo and pray.
Please help.

Rick