Hi,
99 Sport. I dropped my fuel tank for fuel pump business, and pulled/swivelled my tank straps off the tank "by feel"with no visual. They eventually ended up twisting and popping themselves out with little effort.
Could it BE as easy as those square openings there -the straps seem to fit nicely, but it looks like straps would sit too high up to accommodate the actual tank. I'm unable to pick up the tank for some trial-and-error to see where the straps think they want to be in the frame or I would just try that (promise)
Alternatively, I considered that crevice where I have my thumb in pic, but didn't feel anything obvious in there but dirt and rust. I have a tow package if that matters, although I didn't have to un-do anything (but the J bolts) to free the straps.

 

Straps go in the square holes, I don't have the fancy pics some of the other guys do but you get the idea from this one:

 

THANKS!! )