Hi Tom - I tend to agree with Mick that those springs might be a little too stiff. A cyclekart shouldn't weigh much more than 250kg loaded (inc driver) and about two-thirds of that will be on the back end (assuming a rear engine). So probably less than 100kg on the front or < 50kg on each of the front wheel springs (you said "them", so I'm assuming you're not going for a single transverse leaf spring set-up). Using Mick's numbers based on thinning the leaf down to half width or 1 inch wide (which numbers seem right) - that's maybe half an inch of compression for a 1G force. Not a lot of movement!
If I were having custom springs made, I'd definitely think about something softer. Possibly 4 or 5mm thick rather than 8mm, and made only 25mm wide to begin with.
I was wondering whether something like these would be any good if they were cut down to 25mm wide and just the top leaf used:
ebay springs
All the best
Simon