You can put a 200 in as they came out with them as an option. Its more than likely you have the narrow block 170. Finding a narrow block 200 will be close to impossible. The later model (XR, XY etc) 200's are a different beast and are a bit taller if I remember correctly but will fit with a bit of persausion. Ive managed to squeeze a 170 and Borg Warner 35 Auto into my XM, I had to enlarge the opening of the tunnel a bit but a couple of hours with a porta power and it was done. I did have the engine in and out a couple of times to test fit it.
I wouldnt bother with the 2 speed autos, find a BW35 and fit that, the later model all synchro manuals are bigger and need custom mounts. Manual to auto is an easy conversion if you have all the bits, Hardest thing to find is the honey pots, but they are available repro for about $90
Bear in mind you will have to change the coloum and the shift tubes to an auto, and 2spd and 3 spd are different. Have a look at this site to explain it better
http://stormloader.com/falconccwa/columnhint.htm