As long as you are getting into this project, I have a few suggestions.
If you have the early Hundred, make sure that you get the harness that has two OD relays mounted on the INSIDE & the BN1 style wiring to the OD.
The harnesses are actually made in the UK. BW has a few in stock.
I ordered mine with a few internal modifications to wire routing & size. This to accomodate an alternator & a few other things.
The lower run of wiring from the engine back was originally overwrapped with plastic non stick tape. The tape is available from BW.
Get an extra assortment of bullet sizes & sleeves.
The bullets are crimped onto the wires. BW has a nice crimper which works extremely well - #TT-85. With the very high quality crimp, there is no need to solder. If you use a lesser crimper, it might be well to solder also.
The bullets go into the sleeves extremely tight. BW has a bullet inserting tool #SSC1, which makes this job so much easier & the connections very secure.
Put a dab of dilectric grease in the sleeve ends before inserting the bullets. keeps moisture out of the connections.
Once a bullet is fully inserted, it's nearly impossible to remove it. Thus the spares.
It's a really good idea to get new harness hold down clips. The BW clips matched my originals well. Some offerings from other sources don't match.
Lay the entire harness on the floor & very loosely make the bullet connections between sections. Tag every wire end & verify the circuits end to end for continuity before starting to install.
Have a enlarged version of your wiring diagram made so that you can easily follow things. Have it plastic covered & you can check things off with a marking pen.
The main harness where it goes through the firewall may be extremely tight. I had to cut my old harness at this point to get it out. Also, there was no hope of getting the new harness & it's grommet back through this hole without enlarging the hole.
When you go to test things, put a temporary 20 amp fuse in line with the main feed.
It's a lot of work so take it slow & easy.
Hope I didn't rattle on TOO much.
D