not change the whole chassis, I think that's what they meant. Coz your car body is your chassis, so isn't better to just change car?
You will have to to modify you chassis though. You have to cut up some of the flooroard to accomadate the rear drive, AWD or RWD. And after that, you will have more reasons to strengthen your chassis. I've heard people said changing the doors to those of Evo also helps, something like the Evo doors have better sructural rigidity. BUt getting a full set of doors might be too difficult, so a rolcage might be easier.
About the drum brakes, next time take a look at the rear brakes of a normal proton. Thr fronts are disc, while the ones at the rear are the drum brakes.