A battery cutoff switch is standard on school buses. I found mine at an IH dealer, but NAPA also has them. The standard method is to install them in the battery ground lead - that way, if anything goes wrong, or the switch shorts out, its in the ground lead and won't destroy anything. My M37 had the switch on the front of the passenger seat base since the batteries were under the seat, and my 135 had it through the firewall since the batteries were under the rear of the hood. The 135 worked out real nice - the switch was high enough that you couldn't see it unless you stood on the ground and looked up under the dash, but when you knew where it was, you could reach up and turn it easily.
I also installed locking outside door handles - you can buy a standard general purpose truck outside door handle with a key that matches the holes used for the standard military outside handle. Check with a truck utilities company - the folks who build custom truck boxes, etc.