Can't help on the mechanical tach but this system will do:
https://www.summitracing.com/parts/pro-440-426
You can hide the box under the rad support. See attached photo of my 61 Savoy with this setup.
Danny Plotkin
-----Original Message-----
From: "angad22" <angad22@xxxxxxxxx>
Sent: Tuesday, November 30, 2021 6:54pm
To: "Chrysler 300 Club International" <chrysler-300-club-international@xxxxxxxxxxxxxxxx>
Subject: {Chrysler 300} Kit for Electronic Ignition Conversion 1961 Chrysler 300G
Hi Friends,
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Attachment:
Savoy 426.JPG
Description: JPEG image