[Andy S]

Mark why dont you get Russell Derbyshire aka russell ram, to build you what you want, with your spare diff, he's very very good and you wont get ripped off either, contact him through this site.
As for swapping the diff I had to do this last year and like you I have an access panel in the rear bulkhead.
Total time taken for 2 of us from start to finish was 6hrs, and that included nearly an hour farting about after we caught the fuel line out of the tank when reinstalling, and as the tank was almost full it focussed our attention somewhat. So in reality it can be done in about 5 hours no problem, but it does need 2 of you and a couple of good trolley jacks, just take the whole lot out swap the diff over then slide it all back in, much easier on the ground that under the car, just disconnect shocks and brake lines handbrake cable and prop.