Re: Tinsmith's in frame gas tank for cantilever Schwinn.
on my newest copper tank i did a similar spacer method. i've had some tanks not fit exactly right, and have low spots in them when you look at them sideways.
my problem was having the top and bottom keep it's shape with the large flowing curve of the closon frames.
i cut 7 or 8 strips of copper, about 1/2" wide, and folded the top and bottoms over and soldered them inside the tank down the middle, basically making a spine to keep the shape.
didn't bother trying to remove them, i figure if they come loose they're too big to go anywhere, so i could fish them out if i had to.
your tank is more of a box compared to mine, so looks like you wouldn't need a spine...