How do I fix this?

[jlwdvm]

This is the drivers side rear corner of my 69 firebird project. The previous owner installed new full quarters and left the rear tail panel original. The quarter panel lip doesn't fit flush with the tail panel at the seem. The tail panel sits nearly flush at the bottom, and sits in about 1/4" at the top. The passenger side looks ok and I plan to fill in the seem with weld. I would like to do the same with the driver's side, but the uneven corner has me baffled.