This sort of work is not for the Faint of Heart. I used replacement panels from Martin Robey and they were very well done. But, even with well done replacement panels, fit up is crucial. Please spend the time getting the panel to fit exactly before you start to glue it in. If the panel doesn't fit exactly my first suspicion is that there is some other damage to the car which causes the inaccuracy. Look for that first. When you are absolutely sure that there is no other damage that could cause the problem, then question the replacement panel.
Welding it Back Up. If you are serious about this type of work, you need a gas welding outfit and a MIG. I use a Hobart Beta MIG and recommend it highly. Use the smallest diameter wire your unit will accept(.027 works in mine and I seem to have good luck with CO and Argon mix gas). CLEAN IT UP MIGs are fussy about joint cleanliness. If the joint isn't clean, they will pop and fart and generally not do well. CLAMP IT DOWN. Use every clamp you can beg, borrow and steal to locate the panel. TACKWELD FIRST. Try to spread your heating of the metal out throughout the panel by tacking first one corner, then the other, then the next, then the next, then about 1" from the first corner, then 1" from the next, and so on. Continue until you have a tack about every 1" on the seam and then go back and seam or plug weld it up.
Now for the friendly part. If you screw up( for example, I concentrate so hard when I paint that I sometimes forget to close my mouth and have been known to drool into my respirator causing spit to blow out of the respirator vent onto the wet paint or if, as happened to me, your damn cat runs across a freshly painted TR bonnet), just keep cool. Clean the paint off the stupid cat(and, for me, hose out my respirator), have a cup of coffee and wait for the paint to dry-about an hour with lacquer. Sand the blip out and respray(blocksand please). That's it. No one will ever know. Parking dings, gravel whacks, etc-no sweat. Blocksand 'em down and then blow a little paint over them using a spot repair grade of thinner. They are gone. I think my use of lacquer is one of the reasons why I am willing to use my cars as much as I do. It's no heartburn to me if they get a little whacked. I can fix 'em easily. The spraygun I am using is one of those new HVLP guns that are supposed to be more environmentally friendly than the older technology. I use a 5hp single stage compressor on 220volts and it will barely keep up with this gun. I like the older spray technology better but, perhaps that's just because I am not used to the new stuff yet.
One of the Prettiest Butts in the World.
I am Surely Glad Lyons told Haynes and Hassan to Make the Engine Look Pretty.