EFI Prochem Everest heat exchangers. Need information thank you

I’m on my second replacement of the copper HX. Unfortunately for me, I wated to test the waters and see if the copper HX will do the trick, so I replaced all 3 busted HX for copper HX. The HX closer to the exhaust cracked. So I replaced it with copper again. It cracked within a month. I was told the first 2 HX need to be stainless steel. I can’t find them anywhere on the web. The copper part number is 8.602-978.0. Prochem Everest 2006 efi nissan engine low pressure. Thank you