Make sure the drift is softer than the spline, which means it should be aluminium or brass. The awkward part of this operation is that the thread is on a much larger diameter than the spline, so you are hitting the spline much harder than if you were able to apply the torque at the thread's diameter. If you were doing this regularly it might be worthwhile making up a version of B&S tool number 19114. See figure 10 on this page in the overhaul manual:
http://www.tpub.com/content/recoveryvehicles/TM-5-4240-501-14P/css/TM-5-4240-501-14P_82.htmEven if you did that I personally would not use the B&S flywheel holder for this job. Instead I would hit the handle of the 19114 tool with a rubber hammer. However the flywheel holder is useful for other jobs.
If you happened to have a strap-wrench that would fit the outside of the threaded section at the base of the splined part, that might be the best solution of all.
