I'm pretty sure the auto box is bigger than the manual boxes so i'd talk to Maddat and make sure their tunnels will clear the auto box. If you want your car off the road for the minimum amount of time I would throughly recommend getting the conversion kits, as having gone the other route of fabricating the crossmember's and tunnel myself I know how long it can really take. They may seem expensive (because they are) but they will be worth the money when you can bolt the motor in and with a weekends work on the end of the welder your motor and box will be sitting in the engine bay. Especially if you have to pay someone to do the welding for you.
I've also done the 280zx strut swap but would probably go the 1200 struts with s13 silvia brakes if I did it again as a few guys on the nz site have done it and its very straight forward and 280zx struts and brakes are bloody heavy too!. Check out www.datsun.co.nz there is a bit of info on their and all the guys are happy to help with info (or physical help if they are in your area)

Nick