How far beyond the aircraft structure does that boom extend?
Depends on your definition of "structure". The tail cone for example is generally not considered structure.
Interesting to see if there are any associated pitch/boomstrike issues at takeoff and landing.
The max ground pitch angle is determined by the boom attach point, not the end of the boom. Israel's KC-767 design (based on the longer -300) solves the boom strike problem by "burying" the boom in a channel in the aft fuselage structure.