As has been suggested a couple times, you need to unhook the IAC after the engine is at warm idle and take it for a drive. Although spark timing is a major player in idle control, I see no possibility of it giving an 1800 rpm hang on coast-down. Is there an EGR valve still in place on the engine? All the LS swaps I do get it deleted, but in the unlikely event it is still attached to the intake -but the exhaust side is open to atmosphere- it could be driven open and cause high idle.