You're absolutely correct, the tach signal is different. It makes it appear as if the rev limiter is somewhere around 10.5k.
It does not pass 360 degrees though, so it's still perfectly usable. I of course considered the idea of throwing in a 6cyl cluster, but, to me, the mileage being consistent with the vehicle history was more important. If this bothers anyone interested in the car, I'll pick up a 6cyl cluster and throw it in for free (they're super cheap anyways).
Thanks for the compliments!