I think Correl has fallen into one of the regular 'traps' on suspension design here.
This explaination comes from a supplier of track control arms to major OEM's:
"The track control arm, ball joint and lower section of the upright are NOT unloaded as you have presumed although thats how it looks when the vehicle is stationary, a substantial anti-roll bar passes through the track control arm and when cornering considerable forces are applied to try to pull the track control arm from the upright. The ball joint and lower part of the upright must be able to withstand these forces or they'd fail in service."
If that's what is required for the donor vehicle then I'm sure it's not a problem here.
My MX-5 has a very narrow part to the upright in the area you indicated and that has the spring and vehicle load acting on the lower wishbone.
The haynes design and many like it will work fine as they put less load through these parts that would have been the case with the donor vehicle due to their lower weight.
Ash as stated above the Sierra ball joint and most others CAN take the load or it would fail when you go round a corner as the anti-roll bar puts considerable forces through the track control arm, i.e. most of the load from the unloaded side of the car is transfered to the loaded side through the ARB pulling the TCA from the upright.
Hope that explains it BV.