Electrical V8 swap no power to anything

To start with the basics, have you performed a resistance check of all the fuses with a volt meter? A visual inspection is not enough because in some cases, the pop isn't easily spotted with the naked eye. Step 2 would be checking the relays. You could also check the current draw from the battery to see if anything is alive and pulling current. If it's flat out 0A, then something is wrong between either the power connections or the grounds to the battery. You could also test for 12V at the interior fuse panned to see if power is making it into the dash harness.

I've never done one these conversions before, but I suspect something big is the culprit, like a blown fuse or a partially seated connector.

That's great news! Sending a virtual high-five!

Question: Did you use the battery cables from the 5.0 donor car or the V6 cables? I think they are different... just a thought.

If you think the battery cables are the culprit and shot, they are going on 30 years old, LMR recently came out with a repair / cable replacement kit. It's pricey for what it is, but it has everything you need along with instructions on how to remove the old cables and replace them with the new ones in the sub-harness alongside the other wiring that has to be kept. I think there was a youtube video in their email when it first came out some months ago. It might be worth looking into based on how deep you've already gone for this swap and considering the sort of issues you're having.

Just a thought before you replace parts, but @WhiteCobra95 could be right. If it was my car, I would get me the EVTM for your year car, and work my way backwards from the connector on the fuse box you jumped the battery to, following the wiring diagram, back to the battery, to verify where the current stops. Good luck.