My 79 did the exact same thing. I still had some lights etc, very faint but there. it even got the selenoid to click once when I tried to restart.
The battery had dropped a plate and was junk. :banghead
New batter and prob solved.
Check the coil. I had this happen with a '73 Ventura. Driving down the interstate and suddenly no electrical, no radio, lights, hazards, no run, etc. Even dumping the clutch wouldn't restart. Pulled over, popped the hood, scratched head, heard hazards, start blinking and radio come back on, started car and drove home. Two weeks later same thing, this time on base, went to auto hobby shop and asked. Was told to replace coil. I did and it never happened again
Another thought came to mind - the timing chain, especially if it's still original. Pull the dist cap, and turn it over. If the rotor doesn't turn, plan for a new timing set - not just the chain. IF your timing set is all original, it'll have plastic teeth on the sprockets
I had a similar problem with my bronco. Changed everything possible only to find out that the battery was only rated to 600 cranking amps and i needed 1000. When I bought the bronco the previous owner said that someone had stolen the battery out of it and he put another one in. Well he must have just thrown in any old battery. It worked sort of, but it was not doing the job well enough and when I would start it cold it would drain the battery to nothing. Sometimes I would get lucky and it would go. Also you might check your voltage regulator because even if your battery and your alternator are good the battery will not get the right amount of juice to keep it solid. My voltage reg was also bad and I was diving home from work and all lights started to go dim and then wham nothing at all. It had drained the battery dry - voltage regulator was not communicating with the alternator to keep it charged. James
David, even if the rotor turns, it could still be the timing chain. Had a 75 Blazer that died. Pulled the dizzy cap and rotor turned. However, it was pointing to where it should have been. Had the plastic teeth and they had stripped some. Allowed the chain to jump some teeth and was essentially out of time.
So not only check that the rotor is turning but that at tdc on comression stroke, it is hitting #1 post on cap.
:stupid...dollars to doughnuts, I'll bet...exactly what my problem was on my '76 Gran Torino, '79 Bronco and my wife's '84 T-Bird.
BTW, there's another little quirk with the 351M/400 engines. Always make sure there's clearance between the coolant outlet on the front center of the intake manifold and the metal fuel line coming up from the fuel pump. Otherwise, it's vapor-lock time in July/August.