[hemi43]

Here's what I did because I had the same problem. Even though you have a roll cage, it wont matter because your windshield header doesn't seem to be fastened to it.
I used a 10 ton hydraulic jack with a 4 foot extension. I bolted one end on the header using the holes that the top bolts on, and I bolted the other end using the rear mounting holes for the front seat. Make sure you don't have a windshield in because you will break it !!
Start slowly by increasing the pressure on the jack, and you will push the header outwards. There will be lots of springback, and when I did mine I actually moved the header 1.5 to 2 inches outwards before it came back to an acceptable position.
This might sound crude, but it works. My gaps are perfect now!
Trying to fix what you have with shims will be a waste of time.