GM 8 PIN HEI RELAY?

I am going to ditch the MSD 6200 on my Barracuda for the GM 8 pin hei. @TrailBeast offered a kit at one time for this conversion. From what I have found on the internet about his conversion vs. the one on the Slant Six site is the relay. I already have the heavier gauge wires for the MSD box (negative ground and constant positive). Planing to use a relay triggered from the ignition for powering the 8 pin module from the large gauge + and the large gauge for the ground. Where the MSD box was mounted I am looking to mount the module on a heat sink with two relays, one for backup. Is this overkill? or should I just pull the power from the ignition wire source and eliminate the large gauge wiring? Also the coil is a MSD Eliminator SS.

Huh?

Good GAWD. How convoluted. Just run Mopar electronic ignition.

I’d run the big wire.

here is a link to the initial Trailbeast post on the GM 8 pin hei.
GM 8 pin HEI Conversion pics

Many of the internal links are broken, I don't know if the add on modules mentioned in the Trailbeast post are still available.
Concerning the question of adding a back up relay, I don't know the specific relay you are planning to use, but typically relays are very robust and if a replacement is needed one can simply unplug the existing replay and plug in a replacement.
New replacement HEI modules on the other hand, are like new replacement Mopar ignition boxes, available from many sources, some with dubious reliability. I went through two Mallory 4 pin HEI modules operating as a replacement ignition control for a Lean Burn on my '83 D150 in less than a year. After the second one failed I installed an AC Delco 4 pin HEI and it has been fine for six years.
HEI modules need a good heat sink and use heat transfering paste between the two. And they also need a good ground and a good power supply. Since you have them I would use the 'heavy wire' ground and power supply with the power supply coming through a relay triggered off the ignition wire.

and here is a MS - HEI article that explains the features of the HEI modules
General Motors 7-pin HEI Ignition Control

Prior to swapping in a MSD box I ran a seven pin HEI module with a Mopar electronic distributor on a very early version of my 68 Barracuda. The engine in use then was drag race only and was very loose and had some real blow by issues. I would get spark plug mis fires at high load and high RPM. I switched to the MSD 6201 6A box and the mis fires went completely away. Six years and a much higher performance engine in the 68 Barracuda today and I am still running the same MSD box.

Thanks for the replies. I am not very familiar with the gm 4-pin or 8-pin modules conversion. But, from what I have read it is reliable. Due to the fact that the conversion is based on easy to find and replace parts is why I am going this direction. Where I live on the Olympic peninsula auto parts are hard to come by when your car dies in the middle of nowhere, especially aftermarket. I have used the analog MSD 6200 boxes for years, primarily on VW based off-road cars with few problems, when they die it's game over unless you have a spare. I have owned Mopars for 45 years and the factory ignition module besides not being HEI, is junk and fails.

My68barracuda thanks for the links and info. My biggest problem is that I rebuilt the wiring harness from the firewall forward and don't want to open that "can of worms" again. My problem with all of the information is how many variations there are. Classic "more than one way to skin a cat". I am just relying on other peoples experience of what they did and why. That is the reason of my question about the relay, which appears that it is no needed.

The basics of an ignition relay are simple. The path in the ammeter circuit to the key switch, and the ignition "run" (IGN1, usually dark blue) goes back out through the bulkhead from the ignition switch. This then supplies ignition system power, power to the alternator field and VR, and any smog doo dads under the hood, as well as electric choke if used

THAT LINE IS SUBJECT to voltage drop problems and is the MAIN cause of overcharging in these cars

A workaround is to install a relay to alleviate this drop. THIS DROP PROBLEM has nothing to do with the type of ignition used

Electrically cut the IGN1 wire coming into the engine bay, use the firewall end to trigger a bosch style relay, and run a large gauge fused or breaker protected wire from the starter relay "big stud"

Take the switched power from the relay and feed that into the dark blue "engine bay end"

Bypass the ballast resistor and make sure the brown IGN2 coil bypass circuit is intact

You can then actually use the original coil+ feed to power the HEI module and coil+

That makes sense. I have already bypassed the alternator bulkhead connections and went with a volt guage vs. an amp guage to save guard the wiring. Using a fused 5 pin Bosch relay will give me the ability to prevent overcharging from the 90 amp alternator and have constant power to the HEI module.

Thanks.

Where is the voltage sensing coming from? if that is a "one wire" and if you have an oversized charge wire it may not be much of an issue. Nevertheless, you also want adequate V to the ignition/ other underhood "stuff."

The voltage sensing is thru the ignition. The alternator is externally regulated by a pre 67 style solid state unit. The wiring for charging system has been upgraded to a larger gauge and has a fuse for protection. What you recommended previously sounds like the best idea to supply the ignition, regulator and under hood needs.

YUP that is what I'd do