GTSBoy

Just use stock Nissan.

If the yellow line is on the top - the highest part, then there. But I would think it would be a flat gasket, maybe 6mm wide and 1.5mm thick, stuck up in the lid against the inside top.

I never leave a manual parked in gear. Accidental key turning can result in panel damage because there is no neutral inhibit on a manual. The handbrake is there for a reason.

The lid gasket would pretty clearly go in the part of the lid that contacts the top edge of the reserviour......

Maybe. Although it is so much more likely NOT to be dirty. It is much more likely to be a bad solder joint inside it onto the loom plug contacts. Google fixing that. Has to be cut open and soldered by someone who has enough of an idea what they're doing. And then siliconed closed again. Again, maybe. The coils might be fine. It could be dirty/broken loom plugs here too. Swappsies with a known good set of coils is the only/best way to try to pin it on the coils or related wiring.

Fixed that for you?

If you're going to balance the rotating assembly, then you need to balance the whole rotating assembly. In an ideal build, you balance the pistons so that they are all the same weight, you balance the rods so that they are all the same and the piston/rod combos are the same. Then you balance the crank with the assumption that the identical piston/rod combo is hanging off of each. There is no way that you would go to the effort of balancing the crank to suit unmatched piston/rod combos. By the same token, the are any number of engines that have been slung back together with completely unmatched/unbalanced parts. It's all a matter of what you want to achieve. Bazzilion revs? Balance it. Minimise losses? Balance it? Drive it to the shops below 4000rpm 99% of the time? Save some money (by not spending as much on balancing).

Do you know what detonation sounds like? Do you know what lean backfires sound like? Do you know how to google to put it into diagnostic mode and find out what the ECU is trying to tell you?

When do you get the 3 lights? Why not just put it into diagnostic mode and count the blinks? Could be coils. Could be AFM or CAS connectors also, bad solder joints, etc. These sometimes act up as they get warm if they are not in good condition.

Stiffer <> stronger.

No, it's a normal thing for leaving your car hanging on the bloody gearbox instead of the handbrake. FFS.

All the Skyline seats bolt there. The difference in R34 seats (from R32/3) is the front left is at a different height.

A. Is there any length difference between any part of any of them (sedan and coupe, 33 and 34)? B. If no, then use any part from any of them. C: If yes, then whether you can use the front part from a 33 will depend on whether the front parts are the same length between 33 and 34. Given that I was under the understanding that 33 and 34 chassis was essentially the same thing and that sedan and coupe were the same wheelbase, I would be surprised if you couldn't mix and match. Don't take this as a categorical answer though.

Correct choice feels good, right?

I can't see any difference in your potato photos. Completely different angles, etc etc. Supposedly, the difference between the earlier and later RB20DET/RB25DE style boxes was the doubled synchros. I knew nothing about any gears being different.