NewZed

Here's some visual food for thought. Looks like lots of space. Maybe no problem? Found the picture on eBay. I'd share the link but they put way too much stuff in it. There's an extra T. Don't know if it matters.

I'd guess that it is a 1970 - 1977 Z. Could be a bad regulator. How about some ammeter numbers? "Jumping" is not enough. Even better would be some voltage numbers at the battery. A good opportunity to get some practice with a meter. Good luck.

Here's an old halfshaft thread that covers the length, binding, and shortening. Might be the best of what's out there.

The halfshafts are all the same, even the 280ZX shafts. One of the Hybridz members compared a range of years and determined that, but, unfortunately his web page with the data is gone. The 240Z's seem to be slightly narrower between the wheel flanges than the 280Z's, but that has not been confirmed. It's not the halfshafts, it's the cars. The binding issue doesn't seem to be a big problem but you can find threads about people grinding areas of the halfshafts to shorten them at full compression. In short, all you really need is the correct pinion shaft flange. Or a 75 280Z propeller (drive) shaft. Or, maybe there's a flange that you can swap on to your 240Z shaft, see link below. I haven't gone through the actual measurements so don't know for sure if this guy's parts work. Some of his information is wrong, like the part about the 79-83 differentials. 76-83 R200's all have the same dimensions, I have had several across the full range. I've also seen square flanges (78) and circular flanges (76) that fit the same way. Shape does not correlate to fit. Anyway, worth some measuring. Check the bolt hole diameters. Make sure the u-joints will fit. Good luck. https://jagsthatrun.com/products/adaptor-flange-for-datsun-z-v8

Is the R200 from a 1975 280Z? They had an odd flange size. The other 280Z R200's will swap over directly. Your original 240Z driveshaft should work with those. The R180 and the R200 are the same length. The 75 280Z R200 flange is an oddball.

What engine is going in?

You guys went for the troll's bait. Tiny tiny win for zboi.

This distance?

The topic has come up before. Might be something in the attached link. There are probably others. Compare the holes in the head with the holes in the block, and to the holes in the gasket. You might find that the different gaskets will flow the same amounts.

Oops. Looks like it is.

I would use the Engine Electrical and Body Electrical chapters for the diagrams. They are more detailed. I don't see the ignition relay involved in the lighting system.

Here are some links to how to wire up an HEI module if you decided to try it. You can do it all from the engine bay, no need to get inside the old module. It is just unplugged. I think that you might have to join to download the full documents, but the thumbnails have the schematic. Used to be free over there but now there's a charge. There are old posts on the forum about it too though. Good luck. https://www.classiczcars.com/files/file/64-1977-280z-replacing-the-tiu-with-a-hei-module/ https://www.classiczcars.com/files/file/124-instruction-fitting-a-4-pin-hei-module-to-280z-1977-revdpdf/

Have you already bought the MSD box? A GM HEI module will do the job too and be cheaper. The MSD boxes have been known to be difficult. Sometimes they work, sometimes they don't. Sometimes they work but the tachometer stops working. You would want the MSD instructions for the "magnetic pickup". Two wires. That's what the Hitachi system uses. Actually, there are a few tests that you should do to make sure that the pickup coil is working inside the distributor. The pickup coil can go bad. The air gap can be wrong. If it's broken the MSD box won't work anyway.

Clogged exhaust system comes to mind also. Especially if it has a catalytic converter.

Looks like they might be available in Japan. https://www.carpartsmanual.com/datsun/Z-1969-1978/power-train/differential-gear/r180 https://www.google.com/search?q=38154-78501&rlz=1C1SQJL_enUS862US862&sourceid=chrome&ie=UTF-8 https://www.amazon.co.jp/NISSAN-ニッサン-日産純正部品-ADJUST-38154-78501/dp/B091Z23WNM