No, avoid stacking them.
Gearbox is already earthed to the head, via the block, it's all bolted together.
Earth the head to the strut top (spare bolt hole on back of head) and earth gearbox to chassis leg, and a nice neat one from the cam cover stud.
Job done
I think it was a bad earth, but I'm sorted and she runs now thanks for the help!
i just have a thick 4 gauge earth off the gearbox now, no rocker cover one
I would still run more than one earth.
Some light reading
http://www.ek9.org/index.php?threads/helpful-information-on-grounds-very-important.3480/page-2