If you go to the Year One website, they provide ratings for each repro part they sell, called 'YOCLASS'. Highest YOCLASS number being as good as GM NOS, Worst being "trying to get this to fit will give an experienced body shop nightmares". The current crop of front fenders for 68/69 do not have a strong YOCLASS number, and when I called AMES, Year One and Parts Place, all confirmed (in a nice way, of course) there are problems getting them to fit.
Personally, I elected to get good used GM fenders and repair the lower dogleg section, rather than paying bodyshop labor on a Taiwanese part which will not produce as good a restoration.
For the lower front fender patch panels, there are two manufacturers, and supposedly AMES and PY sell the high quality ones, some other vendors sell the ones that do not fit well. I got mine from AMES, the jury is still out on whether they will fit properly or not.
HOWEVER! I understand the 69/69 GTO Quarter panels are finished tooling and incoming to USA in the next month or two. SUPPOSEDLY these use the original GM dies, which is a similar story as the '69 Camaro, '69 Chevelle and '70 Chevelle full quarters which have been GM Authorized to be as good as NOS. I have no reason to doubt this may be true because, at least in bowtie land, people used to get away with charging $2,000 for a GM NOS quarter for a '69 Camaro convertible and now you can get a GM Authorized panel for $399.
Life is good.