Richardson is not a particularly Manx name as you can see by its paucity in the IGI though it does occur, usually associated with English incomers (the name seems fairly common in Lancashire) - however there were very little (if any) shipping that left the Isle of Man bound for America - it is more probable that any stowing away was done from Liverpool - which then raises question how did he get there, did he stow away on the steam packet (possible but unlikely), pay for a ticket or were his family already in Liverpool. As you have probably already checked there are no Benjamin Richardson's in the Manx IGI