go 10 psi....a guy i knew ran 14 (525rwhp) (he had a 93 cobra, with AFR 165's custom grind cam, 14 psi procharger, ect) and one tank of bad gas killed psiton #8 but of course those cobras did have hyperuetectic pistons.....still, i would rather be on the safe side....my dad has an 03 cobra with about 475rwhp and its way to much for the street to handle....you dont need the extra power, stay safe. 400rwhp will be plenty of fun, and you wont end up in a ditch if you let it all out at one time (almost got put in one this weekend.....when that boost comes on, its game on...about 100HP hits the ground at one time and if you got the pedal burried your tires will definatley break traction) first gear in the cobra is sweet but second is insane....as soon as that boost kicks in it becomes a different car....usually end up taking up 2 lanes .

good luck with your build, keep us updated