Seller : "The only reason I think its not a HO 428 car is because you could not get air conditioning with the HO option."
He's leaving doubt. He's misleading the buyer into the possibility that somehow, someway, the car mysteriously has HO manifolds, but the 428HO option doesn't appear on the PHS.
He should have just said "I installed HO manifolds, it's a regular 428 4-speed", and left it at that.
