Teddy,
If you are only doing this for leg room I don't think you will get what you want. Fully back, your seat back should be just about touching the back of the cab. Removing the tank is not going to allow the seat to go back any farther. I am 6'2" and know how you feel.
What it will do is give you extra space in the cab behind the seat.
I know what you are thinking about saddle tanks and I gave it some thought myself but I never found anything so you may have to opt for designing it yourself.
Not sure about your truck,but I have a 64 short bed fleetside so I put my tank in the spare tire spot and will put my spare in the front of the bed behind the driver (there is a bracket that will hold it there).
It is not an easy job but rewarding when done. Good luck in your efforts.
