All good suggestions IMO,
Tho I would suggest that it is all welded together very carefully if fully welding all the RS parts together, what I mean is, be careful of heat distortion when fully welding, as the whole area is critical for the positioning of the rear wishbone brackets, so if that area has even a slight distortion it "could" affect the dimensions that would get "carried" along to the wishbones, & as most of us are using the standard layout on the wishbone adjustment ( lack of it unless using rose joints etc) might be difficult to adjust the distortion out.
Thats just my "two pennies worth"
On my own car, I have stitch welded along the lengths of the RS5 &7a, & RS4 & 7, on both sides, fully welded where all sides of RS8 & 9 meet RS6 & RS10, hope that makes sense
cheers
andy