the 2.5" extensions fit into your truck receiver, but use 2" receivers on the trailer end.
I run a curt at either 36" or 48" depending on what I am towing.
With any tong weight I really recommend Weight Distribution hitch.
because my weight distribution receive is longer than my normal drop receivers I can can run my extension at 36" instead of 48. And the weight distribution really makes a difference. I had a set of 600lb bars laying around. I think the camper rides better with my 7x14 V nose behind it with Weight Distribution hitch then the truck does without the camper.