If you use used parts, inspect all pieces carefully so you won't have to redo the job later. On a D50 I had the long tube wear from the spring riding on it at one point and it snapped in two. The spring itself was fine.
I have tensioned a few springs by holding the case with my legs and feet and twisting the side tab with a STOUT pair of needlenose pliers. It takes a very strong wrist to do it one-handed and getting it to lock in the slot with the other hand is tough. It CAN be done though if you have the strength and will power. If you have a helper, this would be much easier - two hands on the pliers and another to use a screwdriver to pop the tab into place. Since my hands were on the tools, they were safe. I do recommend eye protection in case the tab slips from the pliers, a small sliver of metal may launch.
If this is beyond what you want to do, take the new parts and the case only to your local shop and they may do it at a low price while you wait, or maybe for free if you buy a pack of bags or such (I know my shop would).
)
