" Note the tiny humans wa-a-y back there. ... or i guess we're looking forward?"
If you filled the cargo bay with ping-pong balls would it exceed the plane's gross takeoff weight? I heard that a Guppy could not take off with a cargo compartment filled with ping-pong balls. That should give you an idea about the density of an upper stage (which the Guppies were built to carry).
Wikki says a ping pong ball is 2.7 grams and 40mm diameter.
Using V= 4/3 pi r3 for the ball and volume of the LCF at 1840 m3 (from boeing.com), I came up with 148,254 kg, or 296,508 lbs.
But this figure doesn't account for the packing space between the balls, so the weight would be somewhat less.
This question alone is reason enough to establish a maximum zero-fuel weight during flight testing.
somebody check my calculation