The tail panel meets the quarter nicely at the top and bottom but sticks out in the middle and I'm not sure how to get it to match the quarter or vice versa. My first instinct is to take a piece of 2x4 and flatten the end of the tail panel a bit but this will lengthen the panel and might cause more problems than not. I could try and bump the end of the quarter out to meet the panel and hope I don't get a bumpy mess.So I am not sure how to make the two ends meet and would appreciate some advice from the pros.

Use a lot of vice grips and start at the top then work your way down clamping while you go. Having a second set of hands will prove, well handy. You should be able to use your hands and push hard against the tail panel while a friend watches the alignment and clamps as needed. Once you achieve alignment on one side move on to the other side before welding the first. I'm sure others will chime in too.

I appreciate you taking the time to answer Nevin however the process you're detailing is for closing a side gap.
I was splitting 1x4's for firewood this evening and took a piece about 1 inch wide and gently hammered the quarter to meet the tail pan. So, I solved my problem.