I asked yesterday about swapping the guts of an l-series five into an a-series four, for a 63A, and am still thinking about that.
But I was talking with a friend about it and came up with another idea, that I'd like to run past people to see if there are problems.
I have access to a ton of L-series five-speed transmissions, as a friend has a bunch of dead 510's. L-series doesn't fit an A-series engine because bolt holes are different and starter cutout is about fifteen degrees offset.
If I make an adapter plate so I can bolt the L to the engine, with the transmission tilted, I have a spacing problem that I have to solve between the end of the crankshaft and the input shaft on the transmission.
I can't offset the flywheel backwards, because the starter won't reach it.
I don't want to offset the clutch mechanism forwards because that gets really complicated.
So, what if I get two a-series flywheels, bolt them together, and use the thickness of the flywheel as the thickness of an adapter plate?
The clutch disc is on the surface it expects, the starter pinion is hitting the ring gear where it expects, the input shaft has the clutch disc located where it expects.
The only problem I can think of is that the input shaft won't be long enough to reach the bushing in the end of the crank, and as for that, I can machine a brass bushing that's double-depth, fitting into the end of the crank and extending all the way out to the face of the rear flywheel.

It would weigh a TON.
But can anyone see a reason it wouldn't work?

That might work. A flywheel spacer might be even better. I would machine up a special bushing that fits in the center of the flywheel, rather than extend the crank bushing. Question: will the starter line up with the A-series flywheel?

An obvious problem is if L-series transmissions have different spline patterns than A-series clutch discs, which I hadn't considered. Then I'm back to having to take apart the transmission to replace the input shaft with one from an A-series engine, and I might as well just do the whole gut swap.
But it's an interesting thought experiment.

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?

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

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.

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.

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 ??

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

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.