Thank you for the question.
You will find that if you ask 5 plastic surgeons for their opinions, you will receive at least 7 opinions. Different plastic surgeons will give you different opinions based on their training, experience, and personal preferences. This range of opinions you will receive will potentially be confusing.
I would suggest that you do your due diligence carefully and choose your plastic surgeon wisely. then allow him/her ( after you have communicated...
It's really a matter of opinion, and the opinion that matters most is yours. You should consult with a Plastic Surgeon and familiarize yourself with the size you might expect by looking at photos of women similar to you. 450 cc implants, are, in my opinion, on the large side. But I have had patients with implants that size who want to be even larger! Imlants in the mid to high 300 s is usually what I recommend, but you must be comfortable with your size selection...
What is your goal? Certainly these implants will be out of proportion to the rest of your body. Nonetheless, this may be the look you wish. If you can try on sizers in a non-padded bra, with a top over them, you may get an idea of what this will look like on you (use a full-length mirror).
In my opinion, I always think going smaller is better in the long run. Remember the bigger you go the more they weigh and the more they can go south on you. However, if I selected the size for my patients, I would be out of business. I encourage you to think of lowering into the 300's, but again you have to live with them. Best wishes!