In the past people have made their own longer straps and gone ahead and mounted the tank to the top tube. I've also seen builds where the tank has been mounted behind the seat or a rear rack/luggage carrier or on an extension from the seat post.
I've been able to mount my tank all they way to the neck if thats what you would like. Right now I have mine closer to the seat.
I just happen to have some straps that where longer then the ones that they provide. A good and simple way is to buy some good healthy plumber strap...but you do not want it too healthy because you will need to bend it to fit the frame better. Mine is not the best looking but I care more about function then looks. But if looks is what you are after and the plumbers strap is to ugly looking you can find something similar to plumbers strap but with out all the holes at Ace, HomeDepot, ect.... And drill your own holes.
I also like to use a old inner tube for the tank channel that will help in not scratching the frame and if the tank vibrates less chance of damaging the tank. Be careful not to over tighten or your studs could leak.