Autometer 5291. This is what you need for most GM transmissions that were set up for a mechanical speedometer... Per Autometer's web site...
Most GM transmissions such as TH350, TH400, 700R4, TH200, &TH250's may use the Auto Meter # 5291 speed sensor. This will simply thread onto the transmission in place of where the cable used to go. Some Chrysler, AMC, & Jeep transmissions may use the same sender. To know for sure, look to see how the cable attaches to the transmission. If it uses a large 7/8" threaded nut, that threads onto the transmission, then you can use the #5291.
New/never used.
$15 including shipping to continental US.
(http://i41.photobucket.com/albums/e270/rficalora/MGB%20Parts/AutometerSpeedocabletosquarewavesignal.jpg)