This is a while ago I see, guess its sorted out - but I happened to have a similar problem today. The poly Saddle bush is typically not made to provide space for a bolt that can lock the bracket on the spring in the hole in the axle bracket. Also it is so wide that it interferes with the edge on the hole in the bracket (that is on my estate wagon, Mk2 1977. On the sedan I have for parts there is no such edge which is apparently there to fix the 2 plates together.
It seems one must make something that fits, f.ex a large bolt through the hole in the spring, maybe with a nut in addition to the head. The original bolts for 3 leaf springs will not fit a single leaf spring.
Today I just tried to get the edge to fit in the hole - could not really see if it did, and crossed my fingers and tightened the nuts. Tried to line things up so it looked right.. Will just drive a bit and see if the axle moves out of alignment.