Thanks Steve. Good food for thought, and to see some solid data is great. An extra thou or so might be wise.
In relation to fitting aluminium pistons, my experience is with those from JP. These have a bit of a reputation for nipping up, and despite the advice from the guys at the factory I always get the greatest clearance that I can convince them to give me. They are incredibly reluctant to give as much as 6 thou on a 500 (say 85mm bore) even though 6-8 thou is definitely required on a vintage engine. They are mostly worried that customers will come back complaining of piston slap. Anyway, a friend has Panthers, which use a slipper-style piston, and although a JP piston needs 6-8 thou, a Hepolite piston in the same engine makes do with 4 thou.
It's hard when the pistons are of uncertain age and don't come with a spec sheet!
Cheers
Leon