NewZed

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

There's some good information in this old thread -

Probably worth your time to study up on how the SU's work. They're not your Dad's Buick's carburetor. Air flow is controlled by vacuum. Seems like your "suction pistons" might not be rising fully, choking off air flow. This from the 1973 Engine Fuel chapter -

Don't overlook the fore and aft movement. The bushing sleeves and bolt length and nuts and washers all need to be considered. There are no clear exploded views of how all of the parts there fit together. You kind of have to mental it all together.

What's the bore size of the T3 parts? With a tight tolerance and some grease it becomes more of a bushing type application. I think the original pin is 16 mm. 5/8" is loose. 5/8" = 0.625" = 15.87 mm The lock pin allows the bushings to perfom their spring action with vibration damping, and also keeps the arm legs (?) connected if the nuts come off. If you switch to the PU bushings there's rotation inside the bushing anyway. If I was using a 5/8" bolt I'd probably just safety wire the nuts or create a flat and use a peenable nut. Just for peace of mind. Just some thoughts. A bolt and some threadlocker and you'll probably never notice a difference.

Can't tell from your pictures if the brackets are swapped. The engines have a tilt to them with proper installation. If the brackets are swapped I think that it does bring the left side up, making the top of the valve cover more level. It can look correct but be wrong. The simplest way to tell might be to check the bottom of the oil pan. It should be level from left to right. Hate to say it and maybe, subconsciously, you already know it but it looks like that custom intake manifold could also be the problem. Where did it come from? Good luck.

Make sure you get a 280Z mustache bar to use with that 280ZX R200, it's needed. And the half-shafts if you don't have the clip-in style. Not clear what you're starting with. The other part of your "old posts" problem is parts availability. They're not as plentiful as they used to be. The 5 speeds and the R200's and the required mustache bar are getting harder to find. You can go aftermarket on the mustache bar but it's expensive. Finding parts might be your decision-maker. A junker 280Z might be a good way to get the parts. Good luck.