MSD questions

According to the MSD 6AL destructions, page 7:

http://www.msdignition.com/WorkArea/DownloadAsset.aspx?id=15032386116

you need the 8910 if you had breaker points OR ARE using the original Mopar breakerless ignition module to trigger the MSD using the white wire

you need the 8920 if you are using your Mopar breakerless distributor to trigger the MSD magnetic input, and have eliminated the Mopar ECU

But have you actually tried the tach? Use the tach output (gray)?