Is it small widthwise or heightwise? It might be useful to post a photo. I always get great feedback when I post a close-up image of the problem.
Also, go here: http://www.kadee.com/hocc_athearn_genesis
Toward bottom of the page some conversions for hoppers and other rolling stock are listed. Looks like your best bets might be #148, #178, and "Old No. 5".
-Matt
Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.
I bought Athearn Genesis hopper off from Ebay. I was going to replace
manufacture coupler with an Kadee No 158 whisker coupler, but the coupler
won't fit in the coupler pocket. It's the smallest coupler pocket I have ever
seen. Does Kadee have an coupler replacement for such a small pocket?