2nd keyway is a darned good idea if only for piece of mind at 7 lbs.
Balancer should be one with a steel center. OEM (cast) is going to be a problem.
ATI makes them with 3 bolt chevy pattern or the 7 bolt chrysler pattern. Just depends on what style pulleys you are using.
Don't know where you are but if it helps, keyway cutting for blower hub use is something I do about once a month.
Any perf. machine shop or even a general machine shop should be able to handle this.
Dave Koehler
http://www.koehlerinjection.com