The lower part of the tail extends the fuseelage, but it becomes much more narrow.
Finish extruding the fuselage:
Extrude the fuselage once more, this time to the very tip.
Front view
scale the polygon down to match the blueprint image.
scale the polygon down to match the size of the blueprint image.
move the polygon up to match the contour of the fuselage.
(Orbit SubObject). Then
orbit,
pan, and
zoom the Perspective viewport to get a good view of the rear polygon.
Polygon Modeling panel, click
(Vertex).
click and Ctrl+click to select horizontal pairs of vertices, then click ribbon
Loops panel 
(Connect) to add three horizontal edges and subdivide the large polygon into four quadrangular polygons.


Polygon Modeling panel, click
(Polygon).
If all four rear faces aren’t selected when you switch to the Polygon sub-object level, then
click and Ctrl+click to select them.
Extrude the faces to the end of the tail.
Front view
scale the faces down to narrow the bottom-rear portion of the tail.
Perspective view
Polygon Modeling panel, click
(Vertex).
move the vertex pairs to follow the contour along the lower part of the tail.


