If your looking for a drop in rpm's then the th350 is the wrong choice. The final drive ratio for it is 1 to 1, same as the original transmission in the truck. If you want to drop rpm's by using a automatic transmission your going to need a th200r4 or a th700r4. Using one of these transmissions has a whole set of issues to put in and make work properly. As far as the 216 having enough power depends on many things. If your motor was fresh or one in good tune it would probably handle having it connected to a overdrive (higher final drive ratio) (more speed, less rpm's). If you motor is tired then it would have a hard time with the overdrive.
