I used quik steel on mine I stuffed disolving foam packing peanuts in then the expoy on top. I think your problem is completely unrelated did you use distilled water when you filled your cooling system back up when you did the swap? I have been running my 4.0 head for 3.5 years now and I love it.
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.