Yeah the outputs are the same, the box mounts are different, and you may need to chop the "trans tunnel" a bit under the seat to allow for the extension for the 5th gear.
If you can find a good pair of front wheel arches, they are good to use for the rear for more wheel clearance.
You can use the wider 120 axles by cutting the trailing arm mount off the axle tube and moving further in.
The difference in width with the 120 front end is all in the wishbones, so in theory you could use a 120 r+p front end at standard width is you had the steering arms on the rack shortened. The upright on the 120 has slightly different bottom pin and top bush, but will swap. 120 hub has hubcentric ring, the early ones didn't.