Converting to HEI, need 12v (Ign 12) source

The bulkhead connector from the 73 manual. THE 73 AND 74 ARE PICTURED REVERSED. Check carefully as I'm not sure how this is oriented. You can tell by carefully examining the "keys" to the individual sections

Your "run" wire, dark blue, should be cavity "N" and your "bypass" J3 (Brown) is cavity Q

You need to check that on both sides of the connector, and "wiggle" the connector under the hood