The voltage drop when cranking is likely resistance in the battery cables and/or connections (cable to block, cable to battery posts, ect.)
Replacement battery cables are pretty inexpensive too.
You already proved you get good voltage to the coil under normal (non-cranking) draw (Switch, bulkhead connections, fuseable link are OK.)
A 14 awg wire from the block to the ECU case will help the ECU detect the signal from the magnetic pickup in the distrubitor.