Are you attempting to fit the coupler to the box mounted on the truck? Some designs are extremely difficult, if not impossible to fit.
If you are going to mount the couplers on the body of the boxcar (or whatever), you will need a draft gear box to mount the coupler in. They can be shimmed down or the car can be raised to make the coupler the right height. If you need to change height dramatically, you can get overset or underset couplers.
Browse through the Kadee web site to see all of what is available.
For IHC Rolling stock:
Their recommendation is to convert to body mount using #5 with draft gear boxes:
http://www.kadee.com/htmbord/page47.htm
Their item #10 is a 10 pair pack of #5's (without gear boxes) and #11 is a 20 pair pack. Item #3 is a #5 coupler all assembled in a draft gear box, so you can just glue or screw the entire assembly in place. There is a bulk pack of #5's with the draft gear boxes as well, but I can't find it.
If you want to keep the Talgo-style trucks and couplers, they recommend #27 or #28:
http://www.kadee.com/conv/pdf/i139.pdf
This would be a useful started kit to have, and the price is not bad compared to buying everything separately:
http://www.kadee.com/htmbord/page394.htm
Hope that helps.
Andrew