The grounds are definitely important, and can effect both the sending unit and the fan motors themselves.
As for the “185 degree sending unit”, that’s the fan switch you’re using. Are you sure you have the temperature range on that switch for triggering the fans right? Because turning the fans on at 200 and off at 185 is pretty common for a non-adjustable switch. On at 185 and off at 165 isn’t a common range.
As for moving the switch, it should function just fine in the top of the intake. Moving it to the drain petcock would mean putting it after the water has been cooled by the radiator, so the fans would come on at a higher engine coolant temperature because they’re seeing water that’s been cooled instead of the water coming from the engine before it goes through the radiator. So that doesn’t make any sense at all, because you already think the fans are coming on too late. Putting them in the cooler water after the radiator pass will only delay them further.
***edit***
Ok, maybe scratch that last part. I was thinking of a standard downflow radiator, and I think your Griffin is a cross-flow. So, whether or not the water has been cooled depends on which tank the drain petcock is in. It could be on the engine side, which would be before the radiator pass. Although in that case, moving the switch there shouldn’t change much.