there is a tubular front suspension for sale for b and e-body pretty cheap.
Q is is it worth trying to adapt it to my Duster?
I have the late style discbrakes on it.
Is it only the k-member bolt spacing that differs or is it the upper and lower controlarms also?

There's quite a bit of difference in size of the control arms and a few other things. The design is similar but the parts won't fit at all. If it's for an E body then it most likely only fits the late wide B body but the early is more similar and only narrower. I have seen people weld a whole B body front frame section to an A body and an F body. The tube control arms from the E body won't fit the A body at all.