Read somewhere that hydrolastic top arms have a different ratio that stiffens the suspension when used with rubber doughnuts -- something that Miglia racers used to do? I thought I might try a pair on the back as part of my suspension/brake rebuild but as it requires a bit of faffing about drilling the damper mount hole I though I'd check with you techy folks first -- any thoughts?

You are correct, the knuckle joint mounting is slightly further away from the inner pivot. so yes a slightly higher effective spring rate.
It's not difficult to drill out the shock mounting hole, as there is already what is effectively a pilot hole from the bump stop mounting.

** Ammended due to brain fade on my part **

If the knuckle joint is closer to the inner pivot this will increase the lever ratio and which will lower the effective spring rate and make the springing feel softer.

Ah, yes indeed I must remember to do as I preach and check my answers
It should of course read " the knuckle joint mounting is slightly further away from the inner pivot" [on a hydro arm]

I've ammended the original post.