I have a cj7 with a rebuild 258 engine and a 242 head conversion, can you assess the change in the compression ratio and BHP? (no catalytic converter and computerized emission control).
The 4.0 head swap onto the 4.2 engine is good for a ~40hp/20lbft gain and raises the CR from 8.0 to 8.7:1. It's the single best performance mod that you can do to any 4.2 engine (as well as ditching the single barrel Carter carb in the trash).
If you port the 4.0 head before installing it, the gain could be as much as 50hp. I know that sounds a lot for a head swap but that just shows how poor the airflow is in the stock 4.2 head.
