Hienie had a set of green springs for a shorty. Not sure if he sold them with his shorty earlier this year. SWB springs are different to LwB. In a pinch I have a set of good LWB rear springs to tide you over
Yellow was a very common spring. I bought two sets and used them on both LWB and SWB and they are very comfy. They were known as half ton springs. They do sag quite a bit ( see graph in the link) and i think are more suited to SWB.
I have just been to fetch 400kg of cement with the Chicken Coupé and it handles fine albeit lower at the back.
I am sure you saw that the graph shows loading in Nm (leverage torque or moment unit) which is wrong. It should read N (unit of force ie mass in this case) which you divide by approx 10 to get Kg. The back end of your SWB G with a full tank of fuel and spare i would estimate at 3000N ie 300 kg. The axles are unsprung and can be ignored.
Also remember it makes a huge difference back seat or not. Those are almost 3000N on their own... I have the New Sachs amber (not to be confused with yellow which Sachs have discontinued) and finally happy with SWB springs.