Are you sure you've got a TH400? I don't think I've ever seen one with an electronic speedo drive. A 400 has an oddball-shaped pan with a bulge on the passenger's side. If the pan is square and there's no vacuum modulator, you probably have a TH 700R4 which can have either a gear drive on an electronic pulse type. You can convert the 700 to a gear drive by removing the tailshaft housing and swapping to a worm gear instead of the pulse ring.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!