1. Is the skin where you want it....viewed from the outside? Straight with no visible dents, symmetrical, etc...?
Only after you can say "yes" to question #1...
2. Two bottle jacks, or porta-powers, with 4x4 of 6x6 lumber pressing up against the brace from the floor, each jack pressing approx 1 foot in from each end of the brace...mash the brace up until it touches the skin. But do not go so far as to let it push up on the skin from its natural position more than maybe 1/16"....
3. Hammer and work the the daylights out of each end of this metal brace (being EXTREMELY CAREFUL TO NOT MISS!! If you miss the brace and hit the roof skin, you'll create an "outie" dent to later repair. It will be safer to hammer thru a tranfer device...heavy metal pipe, a solid stick of hard wood, etc...errant hammer blows are far less likely this way since your hammer stays away from the skin) Your goal is to reshape the brace to match the countour it is now pressed and held into. If possibly missing with a hammer and wacking/damaging the roof skin scares you, just skip attempting this whole jacking procedure and proceed to step 5.
4. Release the jacks and see if the roof brace now mostly holds the new shape, with no more than about 1/4" to 3/8" gap. If it's better and just needs a bit more work, repeat step 3. Once you have less than 1/4" to 3/8" gap along the majority of the length of this brace, proceed to step 6.
5. If the brace still has a large gap between it and the skin, cut the brace out, (it can be much more easily re-shaped outside of the car), shape/test-fit/shape/test-fit/shape/test-fit, and when it is shaped properly so that there is less than 1/4" to 3/8" gap along the majority of the brace, re-weld it into place at each end.
6. Run a generous bead of structural bonding adhesive (a 2 part product that is 100% solids and will not shrink at all when curing) to fill any remaining gaps, on both sides of the brace, the full length of the brace. Do not clamp this or try to press it tight...just fill the remaining gap with a solid bead and let it cure like that. Try not to choke on the price of these products...and the price of the applicator gun. Most run $50 to $70 for a small tube (you will probably only need one tube), plus a similar price for a gun. If you know a local body shop they can probably loan you a gun.
After curing, this will effectively bond the roof to the now-properly-shaped brace, preventing 100% of roof movement and oil-canning, and will substantially reduce vibrations/outside noise/etc transferred by the roof skin.
That's the way I'd do it anyway...there are other methods.
