Not sure what you're describing exactly, but adding extra material that close to the pivots will affect unsprung weight by about 1/4 of what you actually add, therefore an extra kg of steel won't really make much difference...
I'm going to leave the gusseting and boxing to the installer, like my subframe swap kit, I'm trying to provide the components you can't just make from scrap metal with a grinder
I did expect the cross bracing might have to be cut (possibly the same for the z31 guys), and I may still be able to redesign the plate to clear, but I need to know what/where/how first

On the plus side, you won't need to relocate the brake hard line, and if you're careful cutting, you could even keep the the bracket that comes off the arm...