I used to have a truck just like this and the starter would get water in it all the time and not work. I used to have my wife turn the ignition switch while I crawled underneath and beat the starter with a hammer, it would start every time, unfortunately when your by yourself this becomes a problem. But it sounds as if your starter is bad or has water in it.

Can you leave the key on run position and jump the starte solenoid? May be bad solenoid?

Sound pretty typical and for a Ford owner to shift to low four wheel drive reverse while going down a hill.

turn the switch, you should hear the solenoid kick out. if not try what mike said and cross a screwdriver on the solenoid post. if it starts the ignition switch is bad. if it doesn't it's the solenoid or the starter. i don't know if they put relays on 78s. it would pay to look for a starter relay too. it should click when you turn the key on but, you will need to have a finger on it to tell.

I have a 1978 f250 4x4 351 modified c6

The other day I had to move it and was heading down hill.

When I started down hill I dropped to 4x4 low and reverse hit the gas and truck died

Tried to start her up and it won't attempt to turn or crank, batteries new a.d starter

All lights work .

Would Tue ignition switch cause the to shut off mid drive and not come back on?