The eng must be cranking to have a valid voltage drop reading. your ground path is OK as it will start when you jump fire from batt to coil. Jumping batt to coil directly lets it fire so the poor connection/continuity is somewhere along that path in the ign2 brown wire "crank" ign circuit. I would (1) jump batt to fusible link terminal in bulkhead with a probe. (2) jump batt to brown wire ign2 "out" terminal in bulkhead with a probe. if it will start doing (1) then the problem is the FL has broken strands or poor continuity on its ends or somewhere back to the batt path. If it wont start in (1) but will using (2) then the problem is in the rear firewall half of the bulkhead or the ign switch/wiring from bulkhead to/from ign sw including steering col pull apart connector (if used in your app). If it wont start in (1) or (2) then the poor continuity is between the bulkhead brown wire "out" terminal to the coil positive primary terminal (loose/corroded terminal/wire in that path)