All the aftermarket tachs I have seen have a switch on the back for 4, 6, or 8 cyl engines. They simply count the number of times the coil discharges in a set ammount of time and use that to display rpm.
Not knowing how the factory chevy tach is set up, I do not know if the curent one could be modded to work, or if it would need swapped out for a V8 one. I'm sure a little google searching could find you the answer.