You will not like my advice, but based on experience with original brackets...
Here is my advice which may be a lot more work then you expect...
You need to start with just the light bar and both brackets. I would not even consider a repro bracket, get an original- but that is just me I guess.
You need to mount the light bar on the riveted in brackets. Your goal is to make the ends of the light bar match the surface on the brackets exactly. You know when things are getting close when the bolts go in easy.
I cold bend both. i used a large diameter piece of pipe I had laying around and a heavy metal bar about 4' long.
Each time I stepped back and made sure everything looked even including the arch of the light bar.
After a lot of work I got stuff fitting good then I moved on to putting the fenders in place.
With the fender on I moved the outer edge of the bracket to get the in line with the edge of the running board.
Now I realize you may have issues with an existing fender you do not want to remove. I know what I say will work, you might be able to figure out a better way for your car.
FWIW, later i bought a pair of NOS fender brackets. I made a jig from the NOS to check the brackets. I was surprised to find what I made correct on the car by eye was almost a perfect match to the NOS.