The Curt Gooseneck Flat Plate # C65500 is 3/8 inch thick and is rated at up to 30,000 lbs when there is underbed reinforcement. To tow a trailer loaded at 18,000 lbs I do recommend reinforcing the 1/4 inch metal with something to give you stability in between your frame rails.
For instance the Curt 600 Series Gooseneck Installation Kit for Chevy/GMC # C60631 is a great choice for the support you need. I've attached some videos to assist. view full answer...