[ck67goat]

That really depends on a lot of factors, such as what are your goals with the car (daily driver, street/strip, full race)? What are your HP goals and what final drive train and accessories will you run on the car. The '79 G body is a relatively light car, so you could not really go wrong with any of the 3 engine sizes since all will make decent power as long as you match engine components and build to go your transmission and rear end appropriately.
Also make sure you get the date and block codes before you choose and diligently research each block to see what exactly they are since you cannot make a blanket statement that a 400 block is stronger. Lastly, what is the condition of each block and the history of each. Are the engines currently running or disassembled? Are they original, rebuilt and previously bored out and crank turned? All these factors need to be taken into consideration to determine how much remaining using life is left in the engine as well as what will best suit your needs.