The aluminum headed 22RE will probably never stand up to compression ratios required to go diesel. The slant 6 head will probably be fine. Whether or not the bottom ends of either one will be able to withstand the compression required to go diesel is the real question. I'm guessing neither one is going to do real well in the long term. Remember GM's experience converting a 350 to a diesel in the early 80s? Didn't go so well.