6 speed would be better than the 5. It'll be easier mount and the shifter assembly should fit if you get the proper generation. Otherwise you'd have to do some modifications to the tunnel.

There's the Legacy GT/STI 6 speed that is shifted with a rod. This trans was used in all STIs and JDM Legacy H6 cars. There's the newer generation 6 speed from gen 4 up that is cable shift which uses 2 levers to operate the gear changes and is smooth to operate. This one was put in the Legacy with H4 in the USDM market. Same 6 speed with different gear ratios was used in the Crosstrek, BRZ and WRX. All cable shift.

2017 was the last year Subaru put a manual in a Legacy and that was the 2.5 models.

Somewhere on here a member posted about putting the 6 speed from a Crosstrek in his manual Outback. I don't remember the year of the car, but believe it was a 3rd Gen. His car was already manual but he wanted the lower gear range.

The problems you will run into is the CAN system in the car. Things will need sorting.

Unless you find a JDM import with engine, trans and computer, you would need to do A LOT of electrical work. And as far as I know, the only JDM combo is the Legacy GT with the STI trans and that car was a 3.0 R series.

Here's another thing to consider. The CVT in your car has a manual mode and the paddle shift. It's super quick with gear changes. It'll hold whatever gear you have it set in until you shift up which is good for long curves, preparation for heavy cornering and speed out of a curve. (Think racing.) It will shift down by itself only when you stop, putting the gear in 1, otherwise it will not shift up or down until you command it. This is handy in multiple ways. You can start from a stop in 2nd to control torque to the wheels when on slick surfaces. No clutch slip from a stop. Takeoff on flat or inclined surfaces is easy. You can drive the car like a heavy sports car and go crazy with it, but I suggest you work on improving the suspension characteristics. Which is cheaper and easier than a manual swap.

You cannot damage the car with paddle shifting. For instance, the computer knows the rev limitations. It will hold a gear at redline rpm. It won't let you downshift if the engine is going to over rev with the lower gear you are trying to hit until the rpm drops in the gear the trans is in. It's not mimicking gear ratios. It's programmed with the various gear ratios. I think a 2018 is programmed with 8. You can shift the trans without lifting your foot. Hold the accelerator to the floor, the computer puts the engine at the peak power band and you shift up down as needed, or want.

This is a video about a WRX with a CVT. Same applies to the Outback; sorta. The Outback is a lot heavier.

I don't deter people from modifications. People thought I was crazy to supercharge an EZ30D. Now it's got a 6 speed and I'm working through electronic and computer control issues to get it to work like I prefer. My VDC is the first gen VDC and the ECM, TCM, ABS and VDC modules all work together to manage the car. No more TCM has created torque signal problems that affect the engine management. I am working it out as time permits.

https://www.subaruoutback.org/posts/6426369/

 

I only see 6MT for the gen. 4 Legacy with the Spec B models. While there are 6MT availible for 4th gen Legacy's, they are only availible for Spec B models. And Im not interest in hunting for scraps or to scrap a limitedly produced model. I did looking around again with some of the help you gave. The 3 best 6MT from what I can see is 5th-gen Legacy 2010-2014, 4th-gen Outback 2010-2014, 2nd-gen Crosstrek 2018-2023. Which are all "cable shift"? I guess. I can look more into that termanology later, I can picture it. Im just not studied on it. Im also trying to think if the various transmision availible for the wrx and brz have any place in the conversation. I feel like they dont just based on their intended use. So with these 3 other transmissions in mind, you imagine they all are going to be somewhat of an electrical project. Im not afraid of this particular challenge. I have somewhat of an electrical and mechanical bacground. But that background doesnt involve cars. So I am expecting the electrical to be the real project. I guess the main thing I dont know is, where to start learing about this kind of stuff (adapting a non component to a preexisting system) I guess is the way I would describe it.

I am aware of all of the JDM options that are availible or atleast will be availible that are very "similar" to my goal. But my JDM interests are separate from this matter. 2004-2007 Legacy Spec B 3.0R 6MT in wagon and sedan bodies. This is something I really want, but Im a bit of a purest about things like this. I really like unmolested Subaru's. But I do also have interests in building something. This specific "project" idea is something I want to do in USDM. And its the one idea I least understand how to pursue.

And I am aware of how CVT drives and how it drives with paddle shifters. I don't hate them I just dont prefer them as something Im proud of I guess. I think Im just along the lines of "CVT isn't really at the heart of Subaru." or atleast it isnt for me. But I know its the future. And Im still highly considering a new generation outback or imprezza for a consistent daily to rely on in the future.

I would love to read about the thread you mentioned as well. Im wondering if you have any advanced search options you recommend to search so I can find it.

I truly appreciate all the information youve written out for me so far.

 

That thread link is me putting a 6 speed in an auto H6 VDC.

The BRZ trans is RWD.

The WRX is a 6 speed, but lower ratio, like 3.9 I think. Depends on the year.

The cable shift is 2 cables, 2 levers and both levers are worked in conjunction to apply a gear. It's efficient and smooth.

Computer communications is going to be the issue.