Bill, do not rely on glue alone to hold coupler boxes. Although this CAN work (any number of old coupler boxes GOO'd to the floors of old models have only come off with parts of the floors still attached), one simply cannot reply on it.
Tim O'C's practical advice to attach the boxes with trimmed screws is also my favored and time-tested method of doing so, and in the occasional install where a thin floor cannot withstand a screw, I HAVE glued the box in place with Barge Cement, but then insured against shearing forces by drilling inserting .020" drift pins (cut from wire and trimmed to length) either through drilled holes through the body of coupler box, or through the ears. I trim flush and ACC the ends of the pins as they project through the floor of the car.
Although I am now, and have long been a proponent and user of a wide variety of sophisticated epoxy glues (in another hobby, antique wood boat restoration), I find little practical use of epoxies in modeling when fine ACC glues of varying characteristics are so commonly available, not to mention high quality, time-proven, and slow-wicking contact glues such as Barge Cement. Especially to be avoided are the ubiquitous low-quality "5-minute" epoxies which will almost assuredly crystalize and fail when you least expect it (there is a reason why high quality epoxies require hours to set up).
The new Kadee coupler boxes with the neat snap-on lids designed for use with their new Whisker couplers lack the side ears, however, a real annoyance when a simple center install is not possible.
Denny S. Anspach MD