how about using a siphon box overflow off the side of your tank, rather than the back?
either that, or drain that tank down and budge it out a few more inches!
you cannot siphon water out one tank into another without controlling the air pressure in one of the two tanks... without controlling the air pressure, the siphon will operate unpredictablly, even if you put a valve on it.
you start a siphon to supply your sump, and you adjust a valve so that the return pump appears to be pumping water out as fast as it's coming in....
scenario a> your siphon is actually running a hair slower than the pump, and the pump drains your sump at 3:45AM and you are awoken to awful noise of pump running dry, or it has run dry and burned out, or shut down and will restart when it cools off.
scenario b> your siphon is actually running a hair faster than the pump, and while you're stuck in a traffic jam after work, your sump has overflowed and is draining your tank onto the floor.
scenario c> you build a sump large enough to handle enough water from the tank to break the siphon ... but now your return pump will dutifully return all that water, plus the water in the sump itself to the tank, and then procede to run dry as the siphon is broken and can no longer drain the tank
none of these are very good! so I'd recommend using a side of your tank for the overflow, or moving it off the wall some more... maybe some applicance moving equipment would be of assitance