I'd give yourself 4 hrs+.

Time clearing Japanese immigration and customs can be lengthy if you arrive at the wrong time. Busses leave every 20 minutes or so. Rush hour traffic in and around Tokyo is what could slow transit. The 75 minute travel time is just a bit excessive if there's no slowdowns en route (I've taken it in the evening with no traffic and it took around an hour).

If you are arriving during rush hour, you could take the N'Ex to Tokyo central (I think 60 minutes travel time, up to 1/2 hr wait for a train, take the local circle line train one stop (or more) to the station for the HND monorail. Travel time for this route would be 90 minutes + waiting.