I have never tried heating and reforming a standard Sullivan tank. I'm not liking the odds on that one--but if you are at the "nothing left to lose" stage, let us know how it turns out. These are made from polyethylene which is a thermoplastic resin, so it should be technically feasible. I just worry about getting a lot of distortion from uneven heating. If I was going to try it, I would put the tank between two board in a vise and try to heat and squeeze to try to keep it straight--kind of stress relieving as you go. Maybe.
On the other hand, they come right out and tell you that you can use a heat gun on the flex tank:
"Flextanks are molded from a special flexible, translucent polymer blend making them easier to fit into tight places and are very durable. They can be reshaped with a heat gun if desired. Flextanks are for glow fuel only."
http://sullivanproducts.com/?s=flex+tankThe 4 oz. flex tank is nominally 2" wide, including the vent bump, in the orientation that we would use. It will squeeze down by an 1/8" pretty easily. (I have one in my hand right now.)
they have a 6 oz. version of same. My Vector will take a 4-1/2" long tank, possibly longer, so you should be able to fit a 6 oz tank. The dimensions in their table do not include the length of the cap or lines. I just pushed the 4 oz. tank into the plane--I didn't bother with a heat gun.
Good luck,
Dave