May I ask something?Is it possible to modify a B707 to a military cargo carrier? I read that 707-320 has 40t load.
That depends on how you define a "military cargo carrier" and we need to make the distinction between "strategic lift" and "tactical lift."
KC-135s as it is already make up the bulk of the USAF's strategic lift capability (all the fuel tanks are under the floor, where baggage would be on an airliner, and the deck is clear for palletized cargo or passengers) and the USAF has a sizable fleet of dedicated C-135 cargo planes (Boeing 717) and C-137 cargo planes (Boeing 707).
What you're thinking of is probably tactical lift, like a C-130. You'd need not only ramps but a strengthened floor to accept, for example, tanks. So, no, you can't. You'd need C-130s for that, or better yet C-17s which blur the line between tactical and strategic airlift.
Most strategic transport of very heavy items like tanks is done by sealift anyways. And seeing as we're Shipbucket...get to drawing them! :p