MSD questions

1971 duster 340 with MSD 6AL ignition installed with aftermarket tach hooked up and working. Bought the MSD 8910 so i could get the stock in dash RPM gauge to work. Well no luck. From what i can read on the MSD site I think i needed the 8920 unit not the 8910. Who has the stock gauge and a aftermarket gauge running MSD working? Can you let me know what you used thanks!