I'd probably find a flat-down box with enough room for a comfortable 450 and enough kick that you have some height, and don't have to worry about only being a few inches over the box... Then get comfortable disastering it, and whatnot.  Basically all I think about when I do them, is that I'm doing a 360, sopt the rail at about 3, and kinda almost shifty your body to 450... If you are able to spot the rail well from the air then 450 shouldn't be much of an issue... I had the hardest time with them because I would always drift to one side or the other, if that's the case then just compensate by taking off more to one side of the jump, or popping to one side, etc...  Good luck man, wear a helmet, hahaha