you have a few options. You will need either an in line Tachometer filter or send the Tachometer out to be retrofitted.
The filter option is cheap and easy, whereas you would need to send the Tachometer to a quality shop such as redline gauge in California.
Build a filter yourself or buy it ready to go. Threads and instructions over at Jim's firstgens.com to make one from Radio Shack parts, or buy one from MSD.