You might want to get a service manual - not a simple job and requires somewhat speciality tools. Refer to injector threads then the valve clearance/bucket thread using the search function.
That head cover gasket should not be leaking so soon after the last injector work - take it back to who ever did it.
They may have cracked the cover or not used the correct sealant/procedure at the front of the cover next to the oil filler. Worth taking off the cover and having a look to see where its leaking from first.
The glow plugs are not located in the head cover so dont need to be removed.
Not sure why you would be given glow plug seals. You should have been given nozzle holder seals - these are for the injectors where the fuel pipes attach.
If the injectors have been out before you will want to replace the fuel lines - they can only be reused once
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2007 SR5 D4D Auto
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