Well, everything was sort of in place for me to get a first order test fit even with my bum leg. Doc says I'm good to do pretty much anything that doesn't stress the break through my new aircast boot, but no running, jumping, sprinting, wrestling, or heavy lifting. Basically I have to be deliberate with my movements and creative with the use of wheels and levers when it comes to heavy stuff. I'm not pushing it, even though this update so soon may appear otherwise. I had all the heavy stuff in place already, so today was just minor manipulation to see how things sit.
The new diff does in fact mate directly up with the original front diff mount, exactly where it needs to be. The bolt patterns are exactly the same and the lengths are almost identical as well, so I will be reusing the OEM Brat front mount for the differential. I may need to figure out a replacement option for the rubber bushings that secure the front mount to the frame, but for now the originals are intact enough. There's a little interference on the tires at the front of the wheel wells which I should be able to counteract by placing the trailing arm mounts correctly in combination with narrowing the wheelbase.
As far as the width goes, one could ostensibly leave that alone and just flare the wheel wells out to accomodate things, but that's not the direction I'm going to take with this build, as I'm going for a more subtle outward appearance.
Lastly for the big rear diff subframe, it is a few inches too wide and will need some trimming and a creative solution to mount to the frame. likely more bracketry. In the spirit of the build I'm hoping to reuse what I can from the Impreza - I bet I can come up with something to allow me to repurpose the bushings that mounted it to the undercarriage of the newer car when I narrow the pivot points governing the rear wheelbase - the plan for which I've sketched in on the photo. Red dashes indicate cuts and shifts I plan to make to the pivots, green indicates the trimming of the outside edge so I can move the frame mount bushings to a place where they will be able to be attached to the frame.
It looks like the original fuel tank will either need some serious modification to fit in the space with the new subframe, or I'll need to fabricate an entirely new cell. I suspect I should be able to find something serviceable that will fit behind the entire assembly and still meet up with the original fill spout. Just need to take some measurements once I've got the new differential subframe into place. It seems easy enough to put a new cell behind the differential, under the bed, but I'm a little concerned that may turn my Brat into a Pinto in the event of a rear end collision. There's also an empty space up behind the cab, but I'm not exactly excited about that placement, either. Suggestions for solutions are welcomed but as usual not expected