I have had this kind of problem before… How to do this?
One way to do this is to use a temporary Lattice modifier to selected vertex group. Then strech the vertices of the lattice, not the ones of your mesh. And when you are happy choose Apply from lattice modifier, and you can delete the lattice and continue with your mesh.
You can use proportional editing with linear falloff.
Also you can use Shear transformation (Ctrl+S in Edit mode)
with the latst version 2.48 you have a new modifier called stretch that
you can try
you can download the latest version here