I'm getting a subglandular silicone gel 450 cc implant and wanted to know if my breast would handle that size... (photo)

Subglandular
450 cc implants would be a big mistake.I recommend a new technique called Breast Augmentation with Mini
Ultimate Breast Lift™.Using only a
circumareola incision it is possible to reshape your breast tissue creating
upper pole fullness, elevate them higher on the chest wall and more medial to increase
your cleavage.Aligning the areola,
breast tissue and implant over the bony prominence of the chest wall maximizes
anterior projection with a minimal size implant.Small round textured silicone gel implants
placed retro-pectoral look and feel more natural, are more stable, less likely
to ripple or have complications needing revision.Implant profile is irrelevant in the
retro-pectoral position since the muscle compresses it.This technique avoids the ugly vertical scars
of the traditional technique, maintains nipple sensation and the ability to
breast feed.Best Wishes,Gary Horndeski, M.D.

Based on your photos I think that the implants will fit.  However, subG placement has a higher Capsular contracture rate and with time they would droop. I would recommend placing the implant behind the muscle. 

Subglandular implants have a higher rate of capsular contracture, and over time, will look less natural in the upper pole due to the sharp demarcation from your chest wall to the implant. A dual plane approach would give muscle cover over the upper part of the implant, the advantages of a submuscular implant, and should allow filling of the lower portion of the breast.  Regarding size, that is best determined in person, but 450cc would probably work.