I haven't done this, but I can tell you that there is indeed a filler plug welded in that needs removed. Didn't look like a big deal to remove, once the top was off. As for the drip edge, I'm guessing you'd have to weld on the additional extension. The additional piece could be cut from just about any 2dr 71-4 B body, or fabricated.

I would also recommend replacing the upper chrome/stainless channel that the weather strip goes in, with one from a 71/2 B-body. Not sure if the 73/4 are the same...but they might be.