ive just about got my car ready for the road, but cant find my header tank cap anywhere

so im going to order a lever type stant cap but what pressure type do i go for? the car is a 400bhp cossie.

also its to fit a spec-r tank which has a big enough cap. does anyone know what the size is for the big caps, so i dont end up with the small type thats on my evo.
thanks