Just divide 1580 by 2.78 and multiply by 3.55, gives you 2017rpm. This is with a locked converter.

If your speedo is out then the signal to the ECU will be out. What you might have to do (regardless of the diff) is get the speedo right, and then get the memcal program looked at the see what ppk constant is used. ppk is pulse per kM, which is the pulses from the speed sensor, could be 2 pulse per rev or 10 pulse per rev. It might have to be adjusted to account for the tyre size and diff ratio so the ECU knows what speed the car is doing so it knows when to lock up the torque converter.
The cam might be too big for a standard converter too.