I am using the adaptor that Mancini sells on my 440 based 511. AndyF makes it. It is a high quality piece. I'm using it to provide a place for a zinc anode to protect the aluminum heads, water pump housing, and radiator. It doesn't stand out, looks wise.
I have got to the point that I realize that parts that are made for our old mopars are going to be expensive, so if I think I need it, I bite the bullet and buy it and don't look back.

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74 Challenger, bought it new. In 1978 I replaced the original 318 with a 446 and 727. Mild cam, Jardine headers, and Holley Sniper EFI.
New engine! 511" RB, Edelbrock Performer RPM heads, Eagle rotating assy, Comp hyd roller cam, Doug's 2" headers.