The former. I don't believe 205/55 and 245/50 should be run together.
Tire size calculator
People have differing opinions, but I think you want to stay within 1% diameter difference with front/rear tires (even on these old cars). Some say you should try to stay close to the stock diameter always, though it is common for both front and rear tires to be larger or smaller to effect gearing.
If you want a wider rear, 255/45 is the way to go, but you will be limited by the width of the wheel if they are stock. I don't believe any non-Turbos came with rear wheels wider than 8".