I bolted a 1/2 GMC axle under mine, after I had Sid lower it 4". It bolted right up to the stock 3600 springs.
I was told, and have read that the 3600 and the 3100 springs are the same, only the axles were different.
All the steering stuff from the steering box to the steering arms is the same from 3100 to 3600.
Read this thread for more info!!