I spent last night making sure it was exactly the right size, and then welded it all up apart from the steering rack crossmember.
I'm not sure what to do with that bit. It seels a bit too thin, as it's made from 16 gauge. I intend to sleeve the bolt holes, to put spreader plates around the contact faces with the rack and where it joins to the main frame, and to gusset the underneath. It just feels like I should use something a bit thicker (manly).
The design is also crying out for inboard dampers! I measured it all up, and it'd fit perfectly. The rocker arms would mount between the top wishbone mounts, and the dampers would go onto the middle of the steering rack crossmember. I can easily add enough triangulation to the frame to ensure that it stays rigid. If I go this route, then it kinda answers my question about the thickness of the rack crossmember.
Hopefully I'll get some time on the car over easter, I'm not going away, but I'll probably end up working for 2-3 days. Bit of a bummer, but at double time all weekend, it has its upsides!