I agree that I would retire his number. I also would not have retired Halladay because I do think there should be a minimum number of years with the team considered or a share of their career. Though if Howard's number is retired that means Rollins, Utley, and Hamels should all be retired as well. At some point one draws a line and Howard is close to whereever that line is.
Preference should clearly be given to players that contribute to World Series winners. In 1980 Schmidt and Carlton are kind of obvious. I probably would not include anyone else. Rose, Tug McGraw, and Greg Luzinski probably have the best cases. Tug had 4.6 WAR that year which is a huge amount for a reliever (92 IP). Luzinski had his best years prior to that and then his career value was bolstered on other teams, so his case is not as good as Howard's but it is also not that far behind.