I assume you will be running the pos cable to the starter and from there you will draw power for the circuits, correct? the same way chevy did it for years except you will use a pigtail cable from the starter over to your mega fuse pack instead of fusible links in the wiring connected to the starter terminal. if thats the case then you will want to mount the mega fuse as close to the starter as possible for the shortest unfused wire from the supply to the mega fuse.
another way to achieve the same result is to draw power for the mega fuse directly from the battery. the later model chevy trucks (mine is a 2015 and I just replaced the battery in early Jan so thats what sparked the thought) come with a battery cable terminal that has an extra lug on the side and that connects to a mega fuse holder connected right at the battery. you could draw power right from the battery that way. possibly an aftermarket supplier could come up with the correct style terminal or cable for your application needs since the GM cable is a little spendy. just a thought. or maybe you could come up with a good used cable and the plastic holder that fits the mega fuse thing from a wrecker. that keeps things tidy. a google search would likely turn up some pics.
https://www.tascaparts.com/oem-parts...cable-23239733