Is it even possible to use the 5MT trans with the Subie engine, without cutting into the rear trunk?

Yes. A couple guys have done it and it’s the way Scott Amenson is doing mine.

How is that being done, if you don't mind?

Watching this and hoping for photos as this is a future endeavor...

https://www.ebay.com/itm/175281786795

In the last picture that is easier to see, those two bolts, make a bracket that would be a correct height, and mount the cylinder off of that, except it would pull, instead of push. Hopefully there would be more clearance back further? I assume they make a cylinder that pulls also?
Or maybe a different shape of cylinder for more clearance?
Would that be an idea?

There are two types of hydraulic clutch setups for the 5mt. The "push" slave cylinder generally requires either cutting the trunk slightly or lowering the drivetrain quite a bit. The "pull" slave cylinder is mounted much lower and does not require the trunk to be cut. @76-914 has a pull slave cylinder on his black car I think. Newer transmissions have push, older have pull.

Sorry but I don't know. I'm leaving all that up to the builder to figure out.

Any idea of the years that the pull were made available?

I THINK 2003 or 2004 was the last year of the pull trans. Maybe dig through Kent's build thread. I think he mentioned the year of the trans in it somewhere.

Mine is and 03 and it's a hydraulic.

I think that’s what Scott will be doing with my car.

You can get either a push or pull hydraulic clutch. I think the cable type was pull but was changed to hydraulic pull quite a while ago. 2004 is pull, 2006 and later (mine) is push. Not sure on 2005 but you can tell pretty easily based on where the slave cylinder is.

Grey Matter- also built by Scott- has the Subie 5MT. Works great. What do you mean exactly by how that’s been done?

On mine- A (Honda?) Hydraulic clutch master is mounted on the firewall next to the brake master, and made to be activated by the stock clutch pedal. Stock Subie slave on top of the trans. Mine’s a pull-type clutch (2002, I believe) but that’s kind of irrelevant.

Does that help?

From the SubaruGears website:

Turbo TY752 transmissions used a pull clutch which features a hydraulic slave cylinder to push the top of the clutch fork towards the engine.
Non-turbo TY754 transmissions produced from 1999 to 2004 generally used a push clutch with a hydraulic slave cylinder of a different design that pushes the top of the clutch fork away from the engine.
Turbo TY754 transmissions used a pull clutch with hydraulic slave cylinder like the turbo TY752.
TY755 and later from 2005 generally all used a push clutch for non-turbo and turbo applications. These featured a hydraulic slave cylinder to push the clutch fork away from the engine.

So you want a TY752 or TY754 if you want a hydraulic pull clutch.