Hi Tom,

I use a TC20II with my D70, and it works fine, I use it in combination with
the 70-200 VR.

You do have to marry up the converter to the lens however, different
converters work with different lenses.

The only loss you will have is f-stops, I lose 2 stops with the 2x, you
would lose 1 stop I believe, but if you regularly use f11 then it wont
matter.

If you still have the instruction book for you lens it will tell you which
converter suits your lens.

Also be warned, some lenses will not take a converter for example the 105
2.8 macro can be damaged by fitting my converter.

Tom:
   Which Nikon 1.4 tele-extender do you refer to?  There are 3 currently:
TC14EII (which is for the AF-S lenses), TC14A and TC14B (which are for the
manual focus lenses).
   Using any of them with the 75-300mm you mention and the D-70 will result
in no auto-focus.  This is because the TC14EII drives the AF-S lens in-lens
focusing motor electronically and has no provision for use of the in-body
focusing motor used with the non AF-S lenses.  The TC14A and TC14B have no
provision for focus drive at all as they are designed for use with the
manual focus lenses.
   Also be very careful when mounting Nikon extenders to lenses as the
front lens element of the extender and the rear element of the lens can
collide with some combinations.  This is especially critical with some zoom
lenses as the rear element travels quite a distance when zooming.
   Some non-Nikon extenders (Kenko Etc.) have the drive shaft to permit the
use of the in-body focus motor with the non AF-S auto-focus Nikon lenses
such as the 75-300mm and others.
   Hope this helps.
Bob