[SAJ]

Thanks for all the helpful info. I removed both perches today. They came out with a bit of a tap and wiggling. There was rust on them and some bright marks on the top tapers, but they were not really loose. A few thou wiggle when just held down by hand but none I could induce with levers etc when cinched up tight again without the shackle in place. I think they are OK and, even though the nuts were really tight (probably 120 ft-lbs at least), I think that was corrosion and they will go up much more snug when I clean lubricate and refit them.
The “new” spring, whilst still clamped with the centre U bolts has 5-1/8 arch and 31-3/8 between eye centres. This is on the outer (loose) limits of the specs I have. The old spring is tighter with 5-3/8 and 30-13/16 inches. Maybe the centre U bolt is flattening the new spring a bit. I will see when I remove it.
Moving on to the axle question, in elevation it has 2 inches drop at the centre, measured from the ends, excluding the small bosses the kingpins fit on top of.
If this is bent it would cause the perches to lean in as suggested.
I am not sure which distance to measure across the perches, but I have 37.2 inches across the bolt centres at the bottom – agreeing closely with Roundvalley
The distance across the perch shackle eyes seems more significant and I have 33.94 inches, with the shackles out (i.e. no spring tension). I will reinstall the shackles and spring tomorrow and measure distances with load on the perches both inwards and outwards and see if they move, but I doubt it. That is, with axle hanging from the spring and wheels off the ground, and the reverse, car body fully loaded and supported on the springs.
That leaves a weak or too-long spring, bent perches, over-bowed axle or… what else?
I wonder if the perches ARE too deep into the wishbone yokes, which is what Jim and Tom suggested – if they have chewed their way into the tapers with fretting corrosion over a long period of time?. There are several spare threads at the bottoms under the nuts and more on one side than the other. I will check in the morning. If so I can turn up hardened tapered spacers to fix this, but the brake shaft housings will then be a little out of line, I suspect.
Columbia, my perches are both forged not cast.
Tom , some bolts are rivet replacements because the cross member has been in an accident at some time and refitted with 5/16 bolts. The more central one is indeed an aftermarket bonded rubber engine mount the car came to me with. I have the proper forged mount and new springs and will fit them while the front is off to see if it improves vibration on the overrun.
Thanks a million again guys. I may have several things adding up to the problem as suggested I think.
SAJ in NZ