Searching for 12v switched power after ditching ballast and installing aftermarket electronic ignition

So not technically an a-body, but I am hoping to get some assistance here since the concept is similar...

I have been going through the harness on my 1977 Aspen wagon, removing parts of it that I don't need, and ensuring that all the wires and connections are good and clean as this is going to be a regular family hot rod in the warm months. I have converted to a Mallory electronic ignition box, so following an electrical service manual (for 1976 unfortunately) I have been able to remove what appears to be wires no longer needed (factory ignition box, exhaust recirculation module, ballast resistor) but I am keeping the factory ammeter in place and working as is. In the process, I have also added a small fuse block in the car to give switched power to my tach, the Mallory box, and my stereo deck so that everything is in one place and nice and clean.

I am looking to add switched power, via a relay, to power the fuse block in the car (pictured below). I am looking for suggestions on where I could pick up switched 12v power without getting overly complicated. I had thought there may be a terminal on the starter relay (pictured) where I could get switched 12v from (all I will be using on this relay is "I" for ignition, "G" for ground, and the main stud to send power to it) but from what I understand none of the terminals would do that. I am planning to use a 40-50amp relay from said 12v switched power source even though I know those 3 accessories won't need that much amperage.. However it would be nice to do it this way from now in case I decide to add other small circuits to it (i.e. line lock, trans brake, etc.) at a later date.

Looking for some suggestions on how to do this, and maybe what others have done. Also, I am currently using the stock monster starter, but may be converting to the late model mini starter...not sure if that would affect any of this or any of my other wiring.

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