Step 1 - Dismantle
Split both boxes, remove all low range components from both and set them aside. (separately!)
Step 2 - Input shaft
Remove everything from both input shafts, including the bearings. I was able to gently hammer the bearing off the L series shaft using a vice as a backing plate, but I didn't try the Forester shaft as I bought a new bearing - 63/28 fully sealed.
To fit the new bearing onto the L series input shaft you need to make up a sleeve, 24.95 id x 28.05 od x 18 - tight enough to gently tap on with a hammer. Fit the bearing on top of this.
When putting the circlip back on I didn't bother with all the fancy spacers that used to be on there - they don't fit now anyway (especially since I accidentally made my sleeve 19.5 long). All I used was the hollow washer and a circlip as pictured. The washer was too long so I had to machine a bit off the end to make it fit. Then just fit the shaft back into the Forester housing.
Step 3 - Syncro cone
The splined section on the main shaft of the forester box is shorter, so you will need to use a lathe to remove some of the splines inside the syncro cone (from the longer side) to get it to fit into position. From memory it was 4.3 mm.
Now you can fit all the L series gear to the end of the main shaft as it should all now fit. Mine seems to have about 0.2mm of play after fitting the circlip, but I don't think it's enough to fuss over.
Step 4 - Syncro sleeve
Because the syncro on the L series is bigger than the one on the Forester it will rub on the ring gear of the front diff. To overcome this remove 1.6 mm from the outside of the syncro sleeve.
