Roy, he has tried the stop in neutral, without getting a stall. I'm wondering if a loose motor mount is letting the engine shift under hard braking in gear and possibly causing a momentary open or short circuit in the primary ignition wiring.
I'd suggest running a temporary jumper wire directly to the coil + terminal and see if the stall goes away with the ignition switch circuit bypassed. If the engine is running a HEI distributor with a remote-mounted coil, some of that wiring could be compromised and responding to engine torque shifting from bad motor mounts also.
Jerry
