Will a 400 CC saline over the muscle implant be okay on my frame?

Instead of sub glandular you should consider subfascial which has many of the benefits of a SubMuscular augmentation without the animation deformity.

Hello,
Without reviewing your history and performing an exam, I can't provide accurate personalized recommendations. However, general recommendations for the implant pocket location is determined by several factors. One factor is how much breast tissue you have. If you have less than 2 cm of soft tissue on pinch of the upper breast pole, it is typically recommended to have the implant placed below the muscle in an attempt to avoid overt implant show, unnatural contour, and implant rippling. Based off the provided information, I would personally choose to place the implant below the muscle (biplanar pocket position) to help avoid the above potential issues. I hope this helps.
Best regards, Adam Bauermeister, MD