Each make of GT40 replica will need different settings. However a good starting point is 26 psi front and 28 psi rear. You may need to adjust up or down depending on how you drive. My car will be different to yours so you need to experiment and see what happens. If the centre of the tyre tread wears out quicker than the edges lower your tyre pressures and if the edges wear out quicker than the centre then you need to raise them. However you need to take into account other suspension setup geometry adjustments and not just tyre wear! Sorry never a simple answer!
Hope this helps