Cutting gaps

While you are doing the body work, don't worry about the gaps until last.  I usually let the filler bridge over the gaps. This makes blocking from panel to panel much easier, especially if it's through a curve. (think a 69 Camaro going from the door to quarter panel)

I then just use a cut off wheel (slowly) to cut the filler open that has bridged over the gap.  Then use a 3" roloc to grind the rest of the filler back to the metal.   Always be careful.