My 1991 Ford f-150 inline stright 6 4.9L engine stopped while driving(moving at 35 mph)
I replaced both fuel tanks, fuel pumps, new fuel filter , and new fuel line from the filter on back to the rear tank.
New distributor cap, rotor, wires, spark plugs, coil, and TFI(on side of distributor).
New idle air control valve. When I replaced all that the truck ran like a fine swiss watch.
I let it sit in a gravel driveway for 6 weeks and it was rainy and damp. Then I took it for a drive upstate for 3 hours and it took all the hills and valleys no problem. But then... the next day the problem came back again on the way to work and I was late. I do not want to get fired! The truck started again but it took 8 key starts to start the engine. It was hard to start, but once it starts it appears to run OK. What is wrong with my truck?

You have done a ton of good things to that truck. Check for condensation inside the distributor cap. Wipe it down with a soft cloth and WD-40--it will help displace any moisture.

As you likely know, Fords run best with Motorcraft or Autolite spark plugs. What brand plugs did you install? What brand of TFI Module did you install? I'm inclined to suspect the Module.