I am trying to find out what size the stock screws are so I can replace them with socket head cap screws without drilling bigger holes. Does anyone know what size I should get?
Almost every screw on the Hasbro pack is M2.6 (and self-tapping rather than a machine screw like a socket cap would be). Rather than trying to get socket cap screws to match the stock holes, I would suggest replacing the rails with ones designed to accept the correct 1/4-20 socket cap screws. Yes it does require drilling the bumper itself but the result is going to be infinitely better than what you are suggesting.