Looking at the Haynes book, the 1300 GT and 1600's have different sized drums in both diameter and depth.
I have drums and rear brake assembly of a non GT.
Is it worth changing up to the larger size, and if so, is it a case of using a new pair of shoes and new drums? Or do I need new springs, hand brake adjusters, back plates, wheel cylinders .. .??

I wouldn't bother changing. The hydraulic circuit will need to be operated though a pressure reducing valve, so with less powerful rear brakes you will need less pressure reduction, and the handbrake will be plenty powerful enough for such a light car.
John

Thanks for the quick reply