I've done this. The wires will go through the scuttle--you don't have to split the grommet. Just work them through carefully.
The scuttle/bulkhead is a good place to start, and work outwards to the headlights, stop and 'parking' lights (maybe save the instrument cluster for last--you might have more confidence and technique). Replace one connector at a time. If the old harness is in good enough shape you can match up the colors, if not it'll be trickier (part process of elimination). Either way, you need a schematic to confirm things--there's a good one in the shop manual. Be careful to make good connections, the bullet connectors can fool you (wouldn't hurt to put dielectric grease in the connectors). It would help a lot if you had a lift (we had a pit). If any doubt test the circuits with a DVM before going hot, else you let the smoke out of the new harness. Make sure all the ground points contact some clean, bare chassis metal. Don't recall if there were bare wire connections--brake switch, maybe--but it wouldn't hurt to lightly tin the bare copper.
Most of the clamps are accessible, except I recall one on the inside of the transmission tunnel. I couldn't get to it until I pulled the drivetrain, in the meantime I just tied the harness leg to the old clamp with a tiewrap.
Last aftermarket loom we got--for a BN2--was perfect.
Oh, disconnect the battery first.