Whoo, finally got round to installing the M90 in my 245 this weekend. It's only been sitting in the shed for 2 years. The box itself fits fine in the tunnel, no hammering required. I used the standard dog-dish flywheel and diesel clutch setup. LuK 623224000 has all the parts in one box, and they're actually Sachs branded parts in an LuK box.
The clutch pivot was spaced out 4mm with washers. The clutch bite is about 30% of the pedal's travel off the floor, which is just where I like it. It's a very light and smooth action, definitely an improvement over the M47, but that could just be due to everything being taken apart, cleaned and greased for the first time in 20 years and 150k miles. I'm using the standard 240 clutch hydraulics, all the way from the pedal to the slave cylinder. The 240 slave works perfectly with the M90.
The shift linkage and the arm that holds the shifter were shortened 100mm and welded, and a short shift kit from Craving Boost was installed. The top arm that holds the shifter also had to have the rear locating pin cut down to fit in the tunnel, and the bracket it locates in was cut down and re-drilled to be bolted to the existing captive nuts in the 240 tunnel. Shift feel is good, but there's a bit more side-to-side slop in the shifter than I'd like, it's actually got the same amount of slop as the 150k M47 I pulled out. I'm not sure what I can do about that as it appears to be in the actual box rather than the linkage, but we'll see.
The rear mount was custom-made for me by Peter of
www.retroturbo.com (goes by 'silveruser' on here) in the same style as the Captain Bondo rear mount seen previously in this thread, it uses a standard M47 rubber mount and crossmember. He also made me a custom driveshaft spacer which lets the M47 driveshaft bolt up to the M90, and modified the M90 shifter to accept a standard M10 threaded gearknob as I didn't like the M90 knob.
The only other modification required was the bracket for the front exhaust hanger which is held on by the bellhousing bolts. It doesn't fit the M90 as is, 10 minutes love with hacksaw and file had it fitting fine.
Now I am going to crack a beer and wonder when I am going to stop hurting from wrestling the damn box up into the car while laying flat on my back on a brick driveway.