Thanks Damon. this info may apply to your van, I'm not sure. Here is what I think I know....
The mounting system for the rear springs on a 500D differs to that on a 500F. The difference is in how the springs are secured and isolated from the car body for noise.
One key difference is in the positions of the top and bottom ends of the springs, relative to one another - that is to say the end of the lower "pig tail" versus the upper "pig tail". I'll call this 'Spring timing' since if one end of the spring is at 12 oclock the other end will be at a different position on the clock.
On a 500F, spring timing does not matter. This is because the lower spring end is retained by a bump on the swing arm, which stops the spring turning, and the top end is retained by a pressed steel cup, which also has a bump. The cup is mounted over a rubber ring bush which in turn sits against a reciever ring on the bottom of the car. This system allows a spring of any timing to be mounted, since the top cup and rubber ring can mount in any position around the clock.
On a D, the system is different. The spring is retained in the same way on the swing arm (although the system used looks a little different) BUT, instead of a pressed steel cup, the top of the spring just has a rubber bush, and instead the reciever ring welded to the car floor has a bump to hold the top of the spring. This means D models can only accept springs that have a certain spring timing - the spring ends need to fit with the stops that retain them top AND bottom.
My problem is that I have bought aftermarket springs that do not have the same spring timing as D springs. This is not a problem on F cars because the upper cup and rubber mounting can me simply turned around a few degrees(or hours on the clock) as required to suit. It looks as though my only option is to cut a little bit off my springs so the timing matches the lower swingarm mount and the upper receiver mount.
I need to see a parts book to confirm this because I don't have the D swingarms anymore. It won't solve my problem, but it would confirm I am right that this is an issue with aftermarket springs in a D regardless of the swingarm model, not a sypmtpom of adding F swing arms (as I have done). It is just possible that the D swing arms have a flat reciever cup similar to that on the top of an F model which allows the spring to be rotated at any point (The F system upside down!) but I doubt it.
Phew...Well done if you understand what I am saying!
Thanks Roger
PS - I know most parts suppliers show the cup and rubber ring as being for D and F models, but I suspect this is out of ignorance. I have at last found one that lists a separate rubber ring for a D which looks quite different to that of an F. Also, I have tried the F model rubbers on my car and the inner diameter is not sufficient to fit the diameter of the hole in the reciever cuo which allows the shock to travel up through the floor of the car.