Yknow, I ran into this problem last year.
First I mocked up the straps in the floor of the trunk pan, without the pan on - it was not easy!
I took some measurements from the framerail.
Then what I did was set the straps down on the floor, set the tank on top an made measurements.
Then I did the same thing upside down.
Hope this gives you some ideas, without a measurement off-hand.
Best,
Dan