[Prowbar]

Nice work on the brackets and reservoir. Never had any issues with coolant overflow.
The V6 takes 8 gallons of coolant (didn't believe it at first until it all pours in). When the engine is cold I cannot see any coolant in the radiator but when the engine is hot it is about 1" below the cap. Anymore coolant and it will blow it out via the hose under the cap.
I always drill a small (.080-.100") hole in the thermostat so air cannot get trapped between the thermostat.

As far as the blinker issue goes, did you check the harness in the steering column for any exposed wiring? Did you have the park lights on when the fuse went out?
Did you check/replace the hood harness?
Do you have a good ground from the hood to the rest of the truck?

IMO, as far as the drivetrain goes, I'd build up the 351 V6 and rebuild the NP435 transmission. They are sweet shifting units in good shape. The close ratio version is the best for the V6. Yours is probably the wide ratio.
Regear the rear end to 3.23 gears using the DoctorDiff gear set or install a Gear Vendors Overdrive.