The field of view (fov) of your glFrustum() call is:
fov*0.5 = arctan ((top-bottom)*0.5 / near)
Since bottom == -top for the symmetrical projection that gluPerspective() produces, then:
top = tan(fov*0.5) * near
bottom = -top
Note: fov must be in radians for the above formulae to work with the C math library. If you have comnputed your fov in degrees (as in the call to gluPerspective()), then calculate top as follows:
top = tan(fov*3.14159/360.0) * near
The left and right parameters are simply functions of the top, bottom, and aspect:
left = aspect * bottom
right = aspect * top
The OpenGL Reference Manual (where do I get this?) shows the matrices produced by both functions.