Why is my bed sitting on my new gas tank?

[72truckster]

I put in my new blazer thank under the bed. Then I put the box back on and the cross rail is sitting right on the tank. More troubling is that it needs to slide forward about 3in, which would make the cross rail right over the filler. It is a long box, steel floor gmc box obviosly going on a lwb 71 chev. The solution I came up with is to notch out the cross sill.

Thanks for any advice.