Crap,thats horrible...sorry to see this. Ive removed a few 1/4 panels,never replaced them.
If your doing the work yourself,remove the one off the parts car before you touch yours,that way you can see exactly where you need to cut and not cut to much off your own car.
Cut as much of the 1/4 panel as you can off the parts car so you have extra metal to cut/measure/work with.
1995 IMPALA SS
for sale 12k obo