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.