[dsraven]

I would say the residual valve could be mounted in either position depending on ease of mounting but, if it were me, I would mount a single res valve next to the master cyl.
the other issue you may encounter is the system being a bit difficult to bleed due to the "P" trap that is in the lines near the firewall mounted valve. there is a low spot there that could result in air being trapped. it may be better to run a small spiral of line below the valve which doesn't allow any line to go up-down-then back up. there are also stainless flex lines available that work well in this scenario because they allow some flex due to cab/line movement etc. you could attach/clamp the lines at the frame then run flex hose up to the valve and back.
running dual lines to each front brake is great but really, for the amount of fluid the calipers require, a single line run from the master that is run up to a "T" that goes to each wheel would also work.