The 1975 and 1976 US spec 4.5s had catalysts mounted under the hood. Not a good idea.If its a US version, an earlier or later model would be a better choice. If its a European version,it would be great. All of the US spec 4.5s had 8.0:1 compression, the Euro cars had 8.8:1. That's quite a difference. The higher compression European spec 4.5 didn't appear in any of the TDMs until 1974.
Regards,
Randy D.

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W.R.Durrance
ASE Master Tech
Mercedes Tech 19 Years