All of these use the stock TH350 crossmember and the stock length driveshaft.  

TH400 or 2004R
At first, I thought this was an easy swap.  I read an article that said you can get the crossmember from any g-body equipped with a 2004R and it will bolt right up to the TH400.  Well, that is partially true.  IF you have a 84 OR LATER G-body, it will work.  But, the 78-83 G-bodies never came with the 2004R.  The frame rail crossmember mounts are different from the latter year models.  The latter year G-bodies frame mount locations that extend further back (toward the rear of the car).  Since the 78-83 models do not have these, you will not be able to use the factory crossmember (unless you make the frame extension piece).  Other options are to either get a crossmember from G-Force Performance, get a "universal" crossmember, modify/fabricate your own, or make a frame extension piece and use the stock TH350 crossmember.   I modified mine by removing the mount pad from the front side of the crossmember, then using 1/4" thick steel plate, welding it to the back side of the crossmember.   I then drilled an elongated hole for the transmission mount bolt to go through.   Not too difficult to do yourself if you have access to a welder.  Otherwise, you will need to get someone to do this.  The TH350 driveshaft will either need to be shortened, or you can use a stock one from a later model G-body that had a 2004R transmission.  Remember, you will have to use the yoke from the TH400 on the new driveshaft.   

700R4

Go to this site for all the details:  http://www.angelfire.com/80s/monte/transmis.htm 

T5 - 5 speed swap

Check out these links for information on swapping a T5 into your Malibu/A-Gbody:
Installing a T5 Transmission  into a 78-88 A/G Body by Marc Hichens
Installing a T5 Transmission  into the Monte Carlo SS by J. Matthew Daugher

T56 - 6 speed swap

Check out John's excellent site where he swapped a LS1 and T56 into his Monte SS:
http://www.montecarloss.com/ls1montecarloss/ 

DriveShaft Issues
If you are swapping a in a different transmission for racing or to back up a more powerful engine, you may need to upgrade your driveshaft.  I got a HD driveshaft with a 3.5" diameter and 1330 series ujoints made at a local driveline shop.  You can go to Denny's Driveshafts to see EXACTLY how you need to measure for your new driveshaft.  They also explain the differences in ujoint series (sizes).