Originally Posted by DSM_Outback
I have used the 257 HG on a EJ257/EJ25D build, intake manifold fit/lined up fine.
The block will be roughly 2 mm less wide with the thin gaskets therefore the manifold won't line up. You can drill out the holes in the manifold though to make it work but why would you want to do so if you end up with a too high CR anyway? Besides, the ports of manifold to head will also be off (by 1mm per side).
Besides those issues you will also have to use the latest belt tensioner setup with the extra idler when using the thin gaskets to prevent belt slack. With the thicker gaskets you can use your original setup.
As for the EM: I agree with Dave, fully. However, you can use the original ECU temporarily but you will have to run waste gate pressure (ca. 0.4-0.5 bar) only to prevent the issues described.