I've never used spring packing on any car that I've worked on. It doesn't seem to make a difference.
Hugh, am I right in supposing your special has reduced camber front spring? If so, what has happened is this:
Flattening the spring effectively lowers the chassis relative to the axle. That means in a normal fitted position the back end of the radius arm will sit too high. You can just force them down to fit, but as you say, this twists the spring and loads up the shackles. A wedge fitted between the spring and chassis will correct the twist situation, but it will give increased castor angle. This might be no bad thing, but excessive castor does have negative effects, like heavy steering away from centre for example. The other way to do it is to lower the front end of the radius arms, either with special end forgings- like an Ulster, or by drop links commonly found in specials. Either way, the geometry stays correct.

Agreed Alan. A wordy exposition (from me) which details the several pitfalls can be found in the thread "radius arm interchangeability" which started on 13/10/13.

Regards,
Stuart

Thanks everyone.
There was no packing on my car when I took the spring off and as long as everything lines up on assembly then I will leave well alone.
If it aint broke don't fix it.
Adrian.

Location: The New Forest