Space wise I can't see that much waste. You could shorten all those print(F( long debug string)) you would save some but not alot.
As you are not using any motors or a motor shield there is no need to use software servo you can just use servo. (This may save some space) worth a try.
As your not taking input from from serial you can drop the use of software serial and use the RX side of serial for the GPS.
There a few other problems in the design which I think will give you problems later such as the use of all those delays and sleeps.
The only thing in the code which would need a change if you where to move to a mega is the use of software serial.
Mark