Hi everyone !

I've been working on this car for quite a while, the roof had a huge dent, It's pretty obvious that the car got crashed years ago and was poorly repaired by a body shop who preferred to use a ton of filler instead of fixing the damage .

So I took the area to bare metal, the dent was so deep and since you can't really get to the back of it because of the metal structure of the roof, I decided to replace the part and weld on a new piece of roof, it's better now, but I think the metal on the roof was stretched due to the crash and/or bad repair job and it's a bit warped . I can push down on it to align it and weld it to the new piece, But I think it would be preferable for it to be aligned before the welding . but there is still the same problem, I can't really get to the back of that part . the new part is perfectly aligned on its 3 other sides though .

The first picture is the gap between the new piece and the rest of the roof, the second one is when I push it down, What do you think would be the best solution to this problem ? thanks a lot for your help !

I always find it hard to diagnose some problems on the internet. Some things you have to see (and/or 'feel') to see what's really going on.
That said, it might be that the roof just un-sprung a bit when cut loose of the damaged area. I tend to think the undamaged area would not be stretched. Without seeing it, though, my advice would be to either use alignment clamps or strong magnets, to lock in all sides to see if it will fit better. It might do the trick. When I chop tops, the 2 pieces of metal often do not have the same curvature. But careful welding and hammer and dolly work afterwards solves the issue. Welding is going to distort your welds, too, so you might be able to take out the distortion with the required hammer work. Just be sure to tack it in as perfectly as you can get it, first.
If the problem is severe, a porta power used judiciously, might help solve the problem. Just go easy with it!

My thought is you may need to shrink the material making up that panel and edge a bit to get it to lay in nice. Now how this is done is for the others to lend knowledge.

Can you back up your pictures to see more of what going on?
Kinda hard to figure out what going on here.

Thanks a lot for your help ! I ended up clamping it as good as I could, and weld everything very carefully, trying to reduce the heat as much as possible, now it's in place, but I got a lot of distortion on the roof, I've block sanded it to show the low spot, What would be the best strategy to use in this situation ? I've been working on cars for a while, but it's the first time I'm dealing with a large and curved panel, so I'm really learning ! Thanks a lot

The roof is mostly flat, very little curvature in it to trap the warpage. It's going to be very difficult for you to fix it. You can "minimize" the heat of welding, but welding WILL shrink the seam. To fix it, you have to use hammer on dolly to stretch it. MIG welding makes a harder, more brittle welds, so it it more difficult to stretch, without cracking it. Sand it down to the level of the surrounding metal, and use hammer on dolly to slowly stretch the weld seam. This would have bee much easier to do during the welding process, as you could keep track of the condition of the roof, as you went along, correcting it, as it warped. Doing it afterwards, it a LOT harder. It is difficult to do without a lot of experience doing tis, especially on a mostly flat panel. The difficulty lies in how much and where to do the stretching, not the actual 'work'.
The key is to ONLY do the weld seam. The weld seam is what is distorting the rest of the panel! DO NOT try to work outside the weld seam, it will only make things worse! The farther away low area probably is not affected by the weld job, so leave that one alone until you get the area around the weld corrected (if you have any luck doing it!)

Thank you very much for all these informations ! I've tried it a bit and it seems to correct the warpage, my problem now is that a huge part of the welds are behind the metal structure of the roof and I can't reach them so it makes things more difficult ..

here is a picture after a bit of hammer and dolly and the second picture is the structure of the roof, hiding the weld ...

You should work on reducing the welds to level with the surface metal. The heavy beads are not going to help you in this process.

A hammer and dolly on the welds will help to some degree but you may find a shrinking disc helpful for any low spots or areas that flex in and out (oil canning).

You can also use a heating tip on a stud welder to shrink the metal but it takes practice.

The metal has already been shrunk from the welding, You have to stretch the bead area.
The only way to do that is to remove the inner structure, Plannish the bead area Stretching it and then weld the structure back in.

Well, the decision comes to: how nice do you want your metal? If you want it better than it is now, you need to cut out the bracing, and hammer and dolly it. Then weld the brace back in. Who cares what the bracing looks like!
And, yes, you need to sand the welds down a bit more. Hammering on a high weld just tries to flatten the weld, being so thick, it resists stretching it.
Pros can often weld a whole seam and then metal finish it. Beginners, even with quite a bit of experience, tend to have a hard time doing it that way. It IS very difficult to know where to do more stretching, after you've done your initial H & D work. Resist the temptation to work outside the weld seam!