The tank lid originally had a fuel gauge in the recess.
I slit the metal with a cut off wheel at the base of the ridge, then removed some material to lower the height.
Then radial slits at the front to help shrink it.I welded a piece of metal into the whole and welded up the rest of it. Plastic filler was applied to clean it up. Not being a master welder I resorted to the "bondo" in as little as needed.
The tank sides got the treatment too. For these I made a template that was appealing to me and then used a cutoff wheel to make the cut. Then I took the cutout from the left side and held it in place in the right side with a couple of fender washers and small nut and bolts as needed. Then using fiberglass "hair" bonded it together. 
Here it is with filler, ready for sanding.
Bondo on the mid joint of frame.
More filler at horns on rear.
And the head with filler.