Back to the OP's question, I had the same issue when I just upgraded to 2500HD. The hooks will fit in the front holes on either side of the receiver if the spring clip is removed.
Also to the person that heated the hooks to spread them out, not sure but you may have changed the load rating on the hooks by heating them.
2016 Chevy Silverado 2500HD Duramax LTZ Z71 Extended Cab, Long bed
2012 Dodge Ram 1500 5.7L Hemi (with K&N Air Intake) (traded)
2012 Rockwood 2109S (Sold)
2014 Cougar X-Lite 21RBS