I got the larger tank to sit just about flush on the GT380 frame. Not perfect, but close enough for me.
Here's what I started with:

Here you can see where I hammered in the tank so it would clear the frame.


This harness was in the way; the tank was resting on it. Rerouted.


I'm assuming this little thing sat on some sort of rubber pad on its original bike. There's one on the 380, but I've removed it because it raises the rear up too much. I'll put something else down that's a bit thinner.

Cut down enough that it doesn't get in the way:

Here's what I ended up with:


Next step is to clean all the paint and rust from the lip at the bottom edges, fill them with body filler, smooth it over, and then paint. I hate the look of that stamped edge.