You need to get the reluctor and speed sensor for a th400. It is a magnetic pickup sensor like the stock speed sensor, but has nearly 3 times as many teeth. You need to have your speedo corrected in the tune since it will read almost 3 times faster than your actual speed if you don't change it. The sensor has 2 wires just like the stock sensor and the polarity shouldn't matter. You can get the pigtail from GM, but I can't remember the part number. The sensors and pigtail came on the 454ss trucks and thelate 80s/ early 90s 3/4 and 1 ton trucks with a th400.
thanks for the link. im actually trying to sell my th400, i really wasnt worried about the speedo but its good info for a buyer. and if it doesnt sell may end up back in my car and for $150 ill hook it up.