Cool, thank you. I may give that a try.

I turned a straight shaft to fit the main caps of an old block and extended it out the rear to locate into the bolt on snout of the bell housing. This kept things inline during preheating and welding. The length was just worked out so the modified 71B input shaft protruded the bell housing mounting face the same amount as the original 56A box.

The throw out bearing remains square to the pressure plate regardless of travel. I cant see any problems with clutch use/slipping. You could dummy it all up on an old engine before you weld the halves together if you had concerns.

So, sikyne, how did you figure out the lateral dimensions, to get the clutch throw so it would still contact the clutch plate correctly?
Did you cut a fake crankshaft or something to get the case lined up with the engine? I'm thinking if I did this, I'd cut something that was a tight slip fit on the transmission intake shaft, that then expanded to the bore size of the block, so they'd be aligned and perpendicular, but I still have to worry about distance between transmission and engine block and ending up with a slipping clutch.
I'm using an A-series engine in a totally different car, with no driveshaft tunnel, so that's less of a concern.

This is a cut and shut that I did. It was also converted to hydraulic clutch.

Swapping internals would not be possible. Using an adaptor plate would be possible but the front of the L series gearboxes is a fair bit larger and getting it to fit in the standard tunnel could be a challenge. The cost of getting one made would be at least as much as cutting a shutting. Unfortunately, any way you do it wont be a one day job. A custom clutch plate or flywheel has to be made. Also a gearbox mount and a tail shaft will have to be made. The shifter will probably also come through the floor in a different place.

Im not trying to put you off. It's good to have a stronger 5 speed in these dattos, but wanted you to know what other things you have to think about.

I would call a machine shop rather than a transmission shop.

Well that is the catch with people like us that are not confident in welding/mechanical. We end up paying full price. If you are not confident, perhaps you should consider talking to a place that will do the chopping n welding. I have learnt that time is money and sometimes you have to lose to win. It may work out viable and hassle free to get it done by a professional. And work out how much you will lose if you stuff it. Not putting you down.. It's only a phone call at a transmission shop, whats the worse they can say "No" or some insane price ??

My reason for being reluctant to cut-and-shut is, basically: I want to keep the car running.
I have a spare engine block I can use for fitment, and I can get a spare non-fitting five-speed, and hash out all the alignment details without the car being down for repairs.
Rebuilding two transmissions into one would take a while.
But cutting up the only functional transmission I have could take quite a while, especially if I screw up my alignment while trying to weld it back up -- indeed, that could take until I find another four-speed transmission. They're not as rare as the five-speed, but at least around here they are not common at all.
Hence lots of enthusiasm for trying to find an alternate way.
And I'm not at all confident of my ability to fabricate the parts with the required alignment. I have an automatic transmission front end, so I have a bellhousing that terminates in a reasonable flat surface, and I've thought about chopping that up and welding that to some other five-speed. That's one of my backup plans. But that gets right back into the difficulty of measuring and fabricating the bellhousing such that the clutch throw is correct. I'm pretty dubious about managing to get that right.

Datsun flywheels are balanced, unlike some Ford and GM unbalanced flywheels. So it won't affect the balance. The extra weight might put strain on the rear bearing? Better to use a spacer I would think, than a second flywheel.

But why bother? Just cut off a couple inches of a stock 4-speed transmission, then weld it onto a 5-speed.

Wouldnt the weight and Inertia of 2 flywheels kill any performance and also affect crank Shaft balance?
Interesting idea but easier to bolt the l transmission and engine in I would think if hoping to all that effort.
Tail Shaft would need shortening as well as an auto input Shaft into the trans

ddgonzal -- my expectation was that, since the starter's bolted onto the engine, and I'm proposing using an A-series flywheel bolted to the crank, there should be no change from stock in the starter-to-ring-gear alignment. Am I missing something?