The coils have gotten bigger with R410A. I have a difficult time believing you'll have a hard time fitting a new air handler in that space. What size unit is being installed? You should be able to access Carrier's website and find dimensions for their air handler (Bryant is Carrier in case you didn't know... so is Payne)
FWIW, Bryant offers a 39 1/2" tall compact fan coil.. Whew! That was tough!
https://www.bryant.com/en/us/products/fan-coils/fz4a/
Dimensions (inches) 22 x 19 x 39 1/2"
BTW... 9 lbs!?
That baby was bone dry!! You go yourself one really good leak! A leak that big should be easy to find... like following the oil back to the Valdez.