Originally Posted by hypernoodle
There are water jacket ports on the head that don't match up to the 4.0 block either.
Just use a 4.0 head gasket and it'll seal up nicely.
I'm sorry but your wrong! The 4.0 head is slightly wider than your 4.2 block thus the water jackets will overhang causing a massive leak.
I have done this swap and its a good mod. What you need to do is fill the water jackets in the 4.0 head with a high temp epoxy ie: quik steel, or JB weld. I filled the jackets with dissolving foam peanuts then covered over with quik steel epoxy. Then I had the head milled flat. If you want to run your carb you will need to make slight adjustments to your intake and power steering pump bracket. It's definaltely not a bolt on and go but its very simple for your average backyard mechanic. Good luck I would be glad to help you with anymore questions if you need.
build thread: http://www.jeepforum.com/forum/f22/skyj-eccentric-build-1467426/ '90 YJ 5.2 magnum MPI, AX15, NP231 SYE, Dana 30 4.56 open/ 8.8 4.56 detriot locker. 1" shackles
2" BL, 4" stretch all on saggy waggy springs.