Are you going off the cluster for the rpms? mine is about 300-400 off. I even verified it with a code reader that gives me live data and it matches my aftermarket tach and Raptor shift light.
if you can use a code reader that provides live data I would check it first.
Next go over every...
Yes you can swap over 243 heads. BUT...... you need to measure for push rod length. Especially if you take heads to a machine shop and have them true up the surface. Highly recommend this if you get a used set. It's not hard to do. There's even a how to help thread in the Tech sticky threads.