Experimenting with an ex-race car to see if I can make it SVA 'friendly'.

I have 40mm of clearance. Obviously I need to find some more height, so, what's the best way to go about it?

It's sitting on 13" wheels just now, so do I just try and find the extra height in larger wheels?

Or...

Would I be better staying with the standard wheels and start getting handy with a C-Spanner on the dampers?

Bit of both I guess. You don't want to go too far with the C spanners as you might go outside the zone that the suspension geometry is supposed to operate in if you know what I mean (just like over lowering a car)

What he said. It's not as simple as screwing the springs down or putting bigger tires on. Both shift the operating range of the suspension. It might be better, might be worse, but it will shift things.

The right way is to figure out with a suspension program what you'll get before you drive it. Putting larger diameter wheels and tires on will change the kingpin offset, for example. The easy/lazy way is to drive it on track for a day and see how evenly the tires wear.

I had exactly this problem as my car started out as a Formula Ford 2000 chassis.
You can only gain so much in the actual suspension movement before you come up against the limit of available movement, hence why I finished up with 18" wheels to get a sensible 4-5" ride height.
If I were doing this again I think it would be much simpler to lower the suspension pickup points on the chassis to gain the necessary height - there's a lot more to sticking a race car on the road than most people realise!