You can use the bigger yoke and your driveshaft, but you may need to shorten it. If you do, take it to a good driveline shop and have them change the weld yoke on the shaft to a 3R style so you can use a standard u-joint. Otherwise you'll be stuck with a conversion joint which is almost twice the cost and not readily available at all parts stores. Your existing shaft most likely has 1310 joints with outside snap rings and the larger yoke for the TH400 has either the stock plastic injection retaining the cups, or if it's been replaced has inside snap rings. To make a joint work between the two you need a 1310 to 3R conversion joint, or have the end of your shaft changed to the 3R (best IMO).