Driveshaft Length

I'll assume you've read this sticky thread:
42RH / A-500 in a Duster with pictures.
Even though it focuses on a later Duster, it will give you some idea of the tunnel and crossmember mods needed to get an overdrive into your car- but keep in mind that the early As have even more restrictive tunnels and a tighter crossmember than the later As, so your mods will need to be a bit more extensive. You will pretty much be fabbing a new crossmember from scratch, so the need to relocate the trans mount is just part of the game.
Moving the engine forward is going to cause all kinds of issues with the oil pan and radiator clearances, and exhaust clearance to the steering column and box, as well as the steering linkage... by the time you modify all that, it probably would be easier to have done a couple of minor firewall/wiper mods to get it to fit in the stock location. JMO.