Repairing roof edge panel

Hi, newbie here, am new to panel and paint, not a youngun, am 54 yrs old but a willing learner, believe in the motto you can teach an old dog new tricks.
I have a Toyota 4x4 dual cab, did a heap of rust repair and a complete respray, all looked good for a while. then:banghead:, the repair on the roof edge cracked on both sides, all through the bog etc, me thinks it was a number of things. Bog was too thick, and welds were onle tacks not complete seam, worried about distortion, of course it means a complete restart, need advice on how to weld a curved section it is on the fold, around 40cms, should I cut the whole section out and weld in a new plate?, and then smooth it with filler?, hard to get the shape one needs, also should I cut the metal to exact size of removal or slightly smaller to allow for heat distortion, and fill the gap with a seam weld?, don't want it cracking again, it is an old vehicle (83) but she is reliable and on duel fuel, motor is not a Toyota, a 3.3ltr Holden red motor. I know it would be easy to get a proper P&P shop do it, but financially it is out of the question, and of course I like the feeling of DIY when it works out of course:grinno:.
Painting won't be a problem, got the gear.
Welder is a Gasless Mig, just need to know from someone who knows their stuff on the correct procedures for joining the sheets together without too much body filler, would a Nibbler be a good acquisition? for a clean cut.
Hope someone can help
Frustrated welder:runaround:
Waz from Australia

i would cut a cardboat template out use a plasma cutter to cut out the shape then grind out the peice about 1/16 inch gap to be able to withstand heat without warping too bad and also with a little more curve than you need then tack it in place with about 1.5-2" spacing between you tacks, you probably know not to get it too hot. Now to answer you question you probably have about why you over curved it. Now you will need to get a body forming hammer and just tap the metal down to where the curve is a little lower than the original, now as for the body work i would suggest fibre strand filler (much stronger than your average filler, and will hold up to flexing) then when you have it sanded down shoot the area with primer, block sand it now look at the primer when you do this the dark spots are low the light spots are high, when you get it all even you'll probably still have a few low spots this is where you need to come back with some piranah glazing putty to even it out, when that cures come back and block sand it with some very fine paper, glazing putty is very easy to sand so be gentle, then prime and paint

i would cut a cardboat template out use a plasma cutter to cut out the shape then grind out the peice about 1/16 inch gap to be able to withstand heat without warping too bad and also with a little more curve than you need then tack it in place with about 1.5-2" spacing between you tacks, you probably know not to get it too hot. Now to answer you question you probably have about why you over curved it. Now you will need to get a body forming hammer and just tap the metal down to where the curve is a little lower than the original, now as for the body work i would suggest fibre strand filler (much stronger than your average filler, and will hold up to flexing) then when you have it sanded down shoot the area with primer, block sand it now look at the primer when you do this the dark spots are low the light spots are high, when you get it all even you'll probably still have a few low spots this is where you need to come back with some piranah glazing putty to even it out, when that cures come back and block sand it with some very fine paper, glazing putty is very easy to sand so be gentle, then prime and paint Thanks heaps people for response, have a good idea now of what needs to be done.
Cheers for the help