[ray_mcavoy]

Unfortunately, I don't think one of the original OD relays will work for this. The OD relays have normally open contacts and one side of the coil internally connected to the contact that's on the +12V input side. So the contacts only close when the other side of the coil circuit is completed to ground. You basically want the opposite (normally closed contacts that open when one side of the coil is completed to ground) to temporarily interrupt power to the HEI.

When I wired up the 3-speed OD trans we swapped into Dad's 66 C10, I didn't want to use an "out of place looking" modern plastic case relay either. The original OD relays I found were rather pricey and there are some horn relays that look similar but don't have a continuous duty rated coil. So I ended up looking through an old Standard Motor Products catalog I had and found a metal case relay with the required coil/contact configuration. I think it ended up being a relay that was normally used for a heater blower (and other applications) in some late 70's / early 80's GM vehicles. Something like that should also work for interrupting power to the HEI and still look right at home under the hood of a 60's truck.