Trouble with voltage drop

[leftybass209]

Having a voltage drop problem with my 72 c10 350.

Engine wiring harness is an m/h harness for HEI. the rest is stock. The wiring for the 100 amp si style alternator has been upgraded to 10 ga. The alternator wiring is 8 ga to a bus bar. Sensing wire to the bus bar. Other wire ties in with the 10ga power wire before it hits the bulkhead. external regulator, and amp gauge wiring has all been unpinned from the harness completely. There is no resistance wire to the distributor, it's a normal stranded copper wire.

I have 14.6 volts running on the engine side, battery. Every check, at every point, is alternator output voltage with no drop. Same voltage running into the ignition, with around a .2-.3 drop running out of the brown 12ga wire to the accessories.

Any accessory i use creates a voltage drop, and that voltage drop includes on the HEI power wire.

My voltage gauge (conversion pulling power from cluster) ticks widely with the signals, and drops to around 12.2 if i have headlights, wipers, blower motor going.

Things that ive addressed
New grounds for core support, battery cable to block, cable from block to frame.
Two grounds off each head to the cab.
two grounds off the frame to the underside of cab.
Two grounds off the frame to the bed.
Each taillight, reverse light, and signal light has a separate ground to fame.
New cluster circuit board
used two different alternators 63a and 100a
used two different ignition switches, 1 original 1 new

I'm running out of things to check without tearing apart the fuse block and checking for corrosion on the connections behind the glass fuses. Could the brown/white wire from the ignition to the fuse block, which ultimately ties to the alternator wiring cause the issue? Alternator charges fine, has a diode in the wiring to prevent backfeed/run-on.

Any help would be appreciated. Sorry for the long-winded post.