Thanks for sharing, Sekwahrovert. Without knowing the load demand on the guy anchor, here are a few thoughts:
1) I would be really specific about the specifications on the guy wire, preferably with an ASTM callout, plus the diameter, and it should have some short of head or j-bolt type of configuration at the low end to provide pullout resistance. I would also specify the minimum concrete embedment. I realize some of this you might have already specified in written specs or elsewhere.
2) You will need to check the anchorage capacity per ACI 318, if you haven't already done so.
3) Based on my own experience, I know the contractor might have a very difficult time getting that cube of reinforcing put together, due to all the intermediate layers. I would recommend putting the mats of each-way reinforcing only on the outer faces of the footing (therefore, 6 total mats of each-way reinforcing, 1 on each face). You can upsize the bars as needed to meet the ACI 318 min-reinforcing requirements. With this large block I'm guessing you don't need the reinforcing to act as any tension resistance. Your contractor will definitely thank you.