custom fuel cell, inner frame rail dimensions please?

[72swb_327stroker_?]

hey all,

i know this has been asked a million times. i am not one to give up without a good fight... but alas, i give!
i have searched for about 3 days now on these forums and cant find one single measurement for the inner frame rails and clearence for a short fleet steel bed.
my friend is going to build an aluminum f.i. tank for me(for a favor) but he needs the dimensions under the bed. the truck is away for a week getting some rest so i have no access to her.
PLEASE HELP!

thanks in advance guys!

any suggestions or experience will be greatly appreciated!