NAPA shows two OEM style hoses and two *universal* style hoses for a 68 C-10 with a bigblock
http://www.napaonline.com/MasterPage...d=7&SubCatId=6
Check the numbers, I have an idea that they sold you the Universal style hose they had on the shelf instead of ordering an oem style hose that should fit.
Outside of factory hoses across the counter at the dealership I have always had pretty good luck with the NAPA OEM style hoses fitting the application. There may be slight changes just to bypass the copyright laws but it should be pretty close to just right.