I'm flying the 727 too, and the 707 is quite similar. There are some differences though, like the hydraulics (because the elevators and ailerons on the 707 were cable-operated). But the fuel-system and the electrics are very similar, you just have one "unit" more per system.

So, if you're a 727 pilot, you sure can fly the 707! Just make yourself clear, that the 707 needs much more runway than the 727 does, and you'll be fine!  ;)