I highly recommend Preferred Doors. I recently purchased a gear and pulley replacement set for my 20 year old Craftsman belt drive 1/2 HP opener. The parts were very reasonably priced and of excellent quality. What set this purchase apart from others was that it included everything needed to do the job, gears, clips, bushings, spacers, grease... They also included very thorough details on how to replace the parts, reset the limit switches test the opener and reset the wireless openers.
I have them on a 8 foot door. They stretch out to 71 inches with the door closed seem to work just fine. I do have a safety cable passing through the springs.