| The rocker with the spring is the closer. They aren't that easy, especially if the head is still in situ.
The easiest way, is to fit the rocker and shaft with the spring in place then lever the tang into place. Make sure you get the tang under the casting, not one of the other rocker shafts.
Access is made easier by removing the adjacent opening rocker and shaft and lowering the valve out of the way. move the piston close to to TDC to stop it falling through the head.
It's the access that makes the job difficult. The inlets are easiest, front exhaust is awkward, rear exhaust is a nightmare with the head in place. | 
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