Thanks for the advice Mike, I'm using a heavy bottom chain on the mooring.

I poured the concrete last night and took the mushroom up to City Island this morning. I think my mushroom weighs only about 170 lbs. currently. To get the weight up I'm going to cut up one of the external ballast I took from the Ariel's keel and wrap the lead chunks with cable ties around the bottom of mushroom's post. The mooring's position is really well protected from weather and the bottom is all soft mud. The mushroom should sink deep in the mud pretty quickly. We're going to set it Saturday morning.