Head Swap 4.2 engine to 4.0 head

[franked]

I have a 78 Cj with Howell EFI, runs pretty good, but I want more power, I understand going to the 4.0 head will give me about 40HP more, which is the HO head, will that work as well, What header do I use in these cases, and do I use my stock manifold. I need to pass CA smog too, so a header has to have a CARB(California air resources board) number. anyone done this, and can tell me what works?