Re: "Papa Smurf" - 1966 GMC 1500 Custom
 

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.

Re: "Papa Smurf" - 1966 GMC 1500 Custom
 

The 351 V6 and my 350 V8 are also running at different temperatures. On the factory gauge my operating temp is right on the edge of the normal line, closer to overheating. I've verified several different ways that around 200-210* is at that zone. I would guess that the V6 operates closer to 160-180*, because my 180* thermostat starts to open when the needle is dead center of the gauge. More heat is obviously going to mean more expansion. I keep my radiator full to the top to prevent oxidation.

My wiring issue is strange because everything is new. And I never had the blinker issue after installing the new column. The issue came about after I rewired the rest of the truck. I added ground straps all over. Battery to body, frame, and engine. Engine to frame, and frame to body. Also added a hood to body. When it blew the fuse while idling, there weren't any lights on in the truck. I did find that one of my switchback LEDs was slightly damaged. I installed some new ones and I'll just have to see if that helps or not. I really should take a multi-meter and try to find the short.

I also agree with you on the V6 and NP435, but I don't think I'm going to do it on this truck. I have my reasons, even if they may be frowned upon here. The V6 I picked up a few years ago will be used in a vehicle some day, but it won't be in Papa Smurf.