Been there done that.
The reason it's leaking is not because of lack of
torque, but to much pressure underneath. Holding the
Do NOT what ever you do try an tighten them down any more.
This is what I did.
I can tell you that I did exactly the same thing, you said
you only lost one washer, I would probably have been OK if
you had lost all the washers. You MUST replace them to
achieve the correct torque. You also have to clear the shafts
where the studs go of all traces of oil or any liquid. Replace
the nuts, in not the studs. I did not replace the studs.
To fix the situation this is what I did.
Drained the system of coolant, remembering to drain the engine as well.
Took the cylinder head off.
Drained the water from the cam followers. I used a straw,
and tried no to swallow the oil.
Made sure the studs are straight. I had spares.
Went to the auto shop a purchased new nuts and washers.
Rinsed the cylinder head with Meths (methanol / ethanol mix) and
Degreased and dried the stud shafts.
Degreased and dried the block head and studs.
I re-used my cylinder head, wiped it with Meths, and dried until there
was no grease.
Finally I put it all back to gather.
Torqued it up, drained the water out the block, re-filled, the
liquids and it fired up the next day. I couldn't be bothered
by the end of that.
Yellow '79 spit wired by a trained marmot