You could use a 1 1/4" pipe thread tap to enlarge and retap the aluminum block, they're easy to find and cheap. Then, depending on what the EGR 5/8 piece you mentioned is, you could 1) get lucky and find a pipe adapter, 2) fab a new piece from steel pipe with the right size male end, or 3) be able to relocate after some plumbing. Problem solved.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick