NewZed

I was going to mention what Sam280Z said about the Charge light. But the diagram shows a parallel circuit with the check relay so that wouldn't be a problem IF the diagram is correct. Current through the check relay solenoid would supply the L circuit and the bulb circuit would not be necessary. Now that you've mentioned all of the other problems, it seems more likely that it could be the cause and the diagram is not correct. Good luck. At least you know that it's power to the L wire that is the problem.

According to one diagram that I have that is the L wire, for Lamp. It supplies power to the alternator windings. That would explain your problem. Now you just have to find out why it doesn't have power. Nissan made things complicated in 1978 with lots of extra relays and gizmos. I think that the best you can do is to follow that wire back. Looks like it gets power through a"check relay" (maybe your e-brake warning light) and also the Charge lamp. The Charge lamp in the meter should light up when the key is on. The colors aren't right in my copy of the 78 FSM but it shows the various connectors. Maybe you just lost a connection. Edit - looks like they only mislabeled the color, WB, right at the plug. It's back to "L" (blue) farther down the circuit. It does indicate a splice to a yellow wire along the way. Nissan used a crimp on to exposed wire to create branches. Likely a stressed point, susceptible to bending and fatigue.

People say that the welding process itself will often help break a bolt loose. The heat and the current. You can spend a lot of money at a machine shop. Make sure you agree on a maximum price. Sometimes they get obsessed and will take a lot of time to get it out, all at their hourly rate.

Get a voltmeter and measure voltage at the plug to the back of the alternator. The T plug. They should both show 12 volts with the key on. Also make sure that plug is making good contact with the pins in the alternator. I had a plug that was loose and was not making contact. Had to recrimp it with a pair if pliers. Make sure that the charge wire to the lug on the starter is intact. It's the white wire that starts at the B terminal on the back of the alternator. And check your fusible links.

The 260Z carbs and manifolds should bolt right up to an L28. People do it all the time, especially when they have problems with EFI. Many L28's don't have the parts or holes in the head for a mechanical fuel pump though, but the 260Z has an electric pump anyway.

The rail and FPR don't have anything to do with the pump. Pump noise depends on how it's mounted and how powerful it is. Start your own pump thread and you can get a pretty good conversation going. This thread is Pallnet's vendor thread. Don't want to clutter it up.

Use the "Single Fuel Rail Fuel Injected" diagram here. Ignore the others, some aren't quite right. Leave the unused inlets on the FPR blocked. The supply is at one end of your rail and the FPR at the other. Tank-(prefilter maybe)- pump-filter-rail-FPR-tank. https://www.aeromotiveinc.com/in-line-diagrams/

That red loctite idea is good for a first try. High strength thread locker. Worst case the nut just comes off and you're left with more grippy threads. Probably pay about $8 for a tube of Loctite. Most of us try to go cheap with the tools at hand. And regret it.

I like the red loctite idea. Another option is JB Weld Steel-Stick. If you use either, stick a piece of plastic wrap over the nub, leaving the threads uncovered, so that you don't glue the nut to the head. And clean the threads of any lubricants. MEK or acetone preferred for cleaning. Filing flat is apparently personal preference, along with channel-locks vs vise-grips. Maybe based on hand strength, who knows. But if you squeeze on the threads they will deform and you'll get limited contact area. Then the pliers will probably slip. Clamp on solid steel flats and you'll get less slippage. In the end though, if you do strip threads off you'll end up filing flats. I just jumped ahead to where the neophyte will probably end up. You'll probably crush the threads, round off the end of the stub in to a cone shape, then wear it down in to a tiny useless mound. Unless you plan ahead and are very careful. Good luck.

A pair of vise grips might do it. File some flats on to the stub so that you can get a good grip. Position the vise grips in a good spot for grip and leverage and get them as tight as you can. If they start to slip at all when trying to turn the stub, stop. Don't destroy the stub. If you don't have the right type of vise grip take the time to go to the store and buy them. Some time and money now will save you a lot of aggravation. Preparing everything for the twist is key.

This guy put a diff together working with a reputable supplier. Might cost more but probably a better product. http://forums.hybridz.org/topic/127169-mfactory-r200-helical-lsd/

Who rebuilt the engine? And has the 5 speed been rebuilt? Good luck.

Get the part number and Google it. Here's a link or you can just take yours out, the numbers are on the side. http://www.carpartsmanual.com/datsuns30/Datsun-Z-Index/Electrical/Electric-Unit-Switch/Section-1/280Z

The 78 system has a lot more parts and connections to get screwed up. You're starting in the right spot, the fusible links. Classiczcars.com has wiring diagrams and factory service manuals. They'll help a lot for troubleshooting, with a meter.

Did you actually check voltage at the pump? Or just assume that it had no power because it's not working? Om the 78's you can disconnect the oil pressure sender and the pump should get power when the key is on. Easier than trying to measure it while cranking. That will at least tell you if the pump circuit is complete.