I'm looking for the clutch spring, underneath the pickup, I've looked everywhere,78 f150, 300, 4 speed

 

Depending on how much pulling power it needs, you can usually find a spring close enough to do the job. If you have an idea what the at rest length is , it makes it a lot easier to help find one that can do the job.
Well equipped auto supply stores might have one and don't overlook hardware suppliers.

 

Try a NAPA or Federated auto parts store.