Good battery, dead truck

[Muzzy5.0]

OK, here goes ..
1972 C10, 307 V8 3 on the tree.
Been trying to chase down a suspected vacuum leak, truck runs OK but will not tickover off choke. Anyway, it cut out the other week and was dead when I tried to restart. Thought the battery had died, so hooked up a jump pack, fired up and continued on. Used it again another day, no power problems.
Left it a couple of weeks, went to start it to carry on investigating the vacuum leak, totally dead. No start, no crank, no lights, no dome light, nothing.
Checked battery, good. Replaced voltage regulator and ignition switch.
Got power across from the battery to the multi splice connection. Got power to the bulkhead connector. Got power into the ignition switch. The “always on” parts of the fuse box are showing 12v as well. BUT, when I switch the ignition to the on position, the “always on” parts go dead …..
What am I missing ?