sounds like you have exceptional access to parts, and know what you are doing
The CD of Parts Manuals is essential
afaic, the 4.2 MKX Adwest and other steering linkage parts are a direct swap for the 420G
The BW8 is very similar to the BW12, but the later trans has a much better kickdown system, they were fitted mainly to S1 XJ… Both share internal kits with a Ford FMX
The problem is the hard parts, they are all slightly different, and many NLA as new parts
I have a couple of spare BW8 for my vehicle
fyi…here is a post I did recently…if you want to understand the steering box these videos are great, same design used in many Landrovers