I'm not sure where the proper location is for that ground lug, but I strongly suspect it should not be on that head mounting bolt. When the cylinder heads are installed on the block, there is a very specific sequence to tightening all those bolts to very specific torque settings. Once set, you should never touch those bolts again unless you are going to pull the heads off. I doubt you did any damage, but I would remove the ground lug from there since it likely a softer metal that will loosen the torque setting over time. You run risk of blowing a head gasket if the seal is lost. I beleive there are some 3/8 -16 bolt holes on the lower portion of the engine block that you could secure that ground lug too. I'm sure one of the guys more familiar with original wiring can tell you where the proper mounting location is...