I realize that this forum issue goes back a few years, but I have the same problem with the tach reading about 1000 RPM high on my '75, depending on the engine speed.

I too replaced the circuit board and filter, and zeroed the tach as described.  When the key is turned on, the tach does go to zero.

I did not realize that the tach was off until I installed EFI, which also has a tach readout.  I wanted to see for sure which was off, so I bought a hand-held tach that you point at the pulley with a bit of reflective tape on it, and this agreed with the EFI reading.  (BTY, this little hand-held tach is pretty nifty and only cost $22.00.)

I have no intention of going back and pulling the dash again (3 or 4 times already was too much!), but I am curious if anyone found the cure for this.