ok, i got a "installed the tach myself" oops to add to this. truck runs like crap, not constant, only sometimes and in no pattern. change the plugs, no change, swap the wires, still no change, cap, rotor, points....still no change. adjust the valves, rebuild the carb, replace the distributor....still no freaking change.
give up, take it to a trusted mechanic. he keeps it for 3 days and cant figure out whats wrong. we rtun a compression check, he goes back over everything i did again. and it still runs like crap. only now its constant.
so, i remove the dist, yep, its stabbed right, no slack in the bushings, springs arent broke, weights are moving as they should, all is great so i put it back in.
whaaa laaa the truck is purring like a kitten. what did i do? duh....i dont know. but look, i forgot to hook the tach back up. shut the truck down, reattach the green wire, and fire it back up. and its running like crap again. then the light comes on. with the truck running, i cut the green wire at the coil connection. the truck smooths out and settles into a nice idle. HMMMMMMMMMMMMMMMMMMMMMM
trace the wire out to find a small rubbed spot in the insulation where in my haste, i put it beside the rubber grommet instead of thru it. thus sandwiching the wire between the grommet and the firewall. when this would make contact, it would disrupt the coil fire causiong the truck to run rough. 8 days of buying new part after new part, hair pulling, cussing, kicking, screaming, and throwing tools. fixed in 2 minutes with a piece of electrical tape.
id be willing to bet that damn piece of tape is still on that 66 dave now owns. LOLOL
Ya know what? When the diagram says "connect to source that is 12V with key ON", and in your haste you connect it to constant 12V THE BATTERY RUNS DOWN GENIUS! 
Thought I'd share my little "oops" for a good laugh.