Welp, if ya really wanna try it..
Take off the tails and get out something to make a template, cardboard probably. MAke the cardboard template fit flawlessly, then trace it onto some sheet metal (same thickness as the body is.. dunno what that is). Cut out the metal and shape it (if you have metal shaping tools, use them, if not... figure something out). Keep on test fitting until it fits in perfectly.
Remove paint down to the metal where it's gotta be tacked in. Tack them onto the quarter panel, and remove the license plate plastic section. Now make a template for that and do the same thing until it fits in perfectly.
If you're going to put in LED tails or something now is the time to cut the hole/ fab a pocket for it/ weld in the pocket.
Once you're happy with how it all fits together, start welding it together tack by tack. Jump around a bunch and take your time.. the metal will warp if it gets too hot, so go sloooow.
Once it's all welded up, cut the beads smooth and start body work... if you did a greaat job of welding it up you won't need much filler. Use discretion, you don't want thick filler on there, if there's big dips beat them out and use filler to final finishing. Once it's smooth and blocked out prime it, paint a light black guide coat, block again to make sure there's no low spots, and when you think it's perfect, paint it.
I'm not a body man, but that's essentially how I did my doors... this is the same thing, just more work, more difficult, etc.
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