i bought the msd tach adapter. you might be able to get an aftermarket one and save a few bucks, but i'd rather get the one your susposed to and know it will work.
you need the tach adapter, it is not to just run your tach it is needed to run your car. it amplifies the ignition trigger signal so your injectors and other fuel injected related parts will work. without the tach adapter your car will not have a strong enough signal to run. the MSD part number is 8912 if you buy it the unit should just plug right into the dis2 harness, it took like 5 minutes to install it.