Welcome
Not sure I know what you mean by how far down.
However I would say replacing the broken plastic wheel wells is worth it it if you are disassembled to that point. The wheel wells are what stands between your trailer and the road. The better and stronger the wheel wells possibly the less damage to the interior of the trailer in case of tire blow out.
It looks like new ones are for sale at High Sky RV Parts for $400.00 for two. They may not fit for vintage Airstreams.
The drawing photo is from Inland rv and shows some measurements as an example. The shiney aluminum example is from VARR restoration company and the black one is from Out Of Doors Mart.
Google
"AirForums Wheel Well Replacement or Build" for several examples.