There's useful advice available on the internet about fitting the Dapol NEM coupling pocket conversions to various N scale rolling stock. It seems you just need to be brave!
http://www.modelrailforum.com/forums/index.php?showtopic=25799
and here...
What's not totally clear is how to gauge the distance to set the pocket from the buffer beam. The Dapol kit gives you a jig which is OK if you are glueing a pocket onto a simple body. Bogie conversions aren't so simple! Most of my conversions have resulted in a long coupling distance... but in fairness, not too different from what the Rapido couplers provided in the first place. There are short versions of the couplings available - I may need to dabble with that eventually.
I've converted the bogies for the bogie oil tanker train and container train. The bogies design was identical. Simply cut off the old pocket, leaving the rear vertical plastic part; then glue the Dapol pocket conversion onto that with epoxy.
The first stage of the conversion simply fits a rectangular carrier. There is then an insert to be added. It is important to get this the right way up: it should allow the coupling to bend upwards (for which it has a tiny strip of nylon as a "spring") but not allow it to bend downwards. It it bends down, it will snag onto the uncoupling magnets.