Your shift lever probably looks something like the one shown.  I need to know the length of the threaded portion A, as well as the thread pitch.  I may have the OEM files for thread pitch, so please include the make and model year.
Measurement B is the distance from the end of the threaded portion to the bottom of the shift knob.  
Measurement C is the diameter of the shift lever at the base of the knob.  This dimension is critical so the threads on the inside of the knob will engage, but should not be too large so the set screw in the knob can be secured properly.
Measurement D is the total height of the shift lever measured to the top of the boot.

In addition, please provide the dimensions of you stock knob, and if I should duplicate these in your custom knob.  Should the top of the new knob be higher or lower?   Measurements are best taken with a caliper.  If you can, sending your old knob would help a lot.

You can contact me at:   drautox@comcast.net