Advanced Design, where did you mount your ECM?

[50 Chevy LS3]

I'm in the process of laying out what holes I will need in the firewall, and where to mount my ECM, boxes. I have the F.A.S.T., controllers. Will also add the TCI trans. control for my 4L60E.
I'm thinking about putting everything under the original bench seat. The battery will remain in the original, under floor location. Then running the main harness underneath the cab and to the engine. F.A.S.T. says to run their supply and ground wires directly to the battery, and they already have fuses in line.
The rest of the trucks elec. system will be from a fuse box under the dash, and through the firewall, as original. I'm using American Autowire, Classic Update for that.
Any ideas or advise, appreciated.