I'm in the same boat with you Sanebo. I have been trying to wrap my head around a 4.0 conversion for some time now. So far all I've come up with is questions.
Having a spare engine means I don't have to rush. I just want to make sure all my ducks are in a row before I purchase a 4.0 head. The eagle is an 83 SX4 with a three speed auto. What I have so far on the wish list is as follows: New head, headers, clifford intake, and a weber carb.
Some questions for anyone who knows.
1) What are the proper rocker preload specs?
2) What push rods will I be using? 4.2 or 4.0
3) Are there any major (or minor) changes that will need to be made to the engine block or engine components to ensure a proper balance with the 4.0 head?
4) Are there any engine rebuild spec changes that I should be aware of as of a 4.0 head swap?
Sorry all. That was more then a few questions.
Best,
Will