Hello Will,
The Civvy and Military firewalls are pretty much the same on the 2A, so no worries with using your good fwall, make sure its a 4cyl one though, you will have to drill a few more holes, move the dip switch and a few other little jobs, there was a change during the 2A fwall production in 1968 the wipers changed from been mounted on the windscreen to been moved into the LH side of the firewall glovebox and operated by cable, easy to tell them apart as there are extra holes just below the drip rail on the later fwall, and also make sure your good one is not a series 3 fwall as they are very different.
hope this helps.
MC