In my opinion you would be better off with a location that is not on the center of the belly.
A xponder antenna transmission efficiency drops off when it starts to get covered with Oil, dirt and other crud. This is what covers anything located on the center belly.
I put them slightly forward of the F-705 bulkhead between the two outboard most seat ribs using a location that is still an easy arms reach when the seat pan is removed.
Performance is good and it stays clean.