Your only going thru the mat and then 3/8- 1/2 into the metal or plastic so 3/4 long screws would be fine but longer won't hurt. If you have to remove the body panel connectors, those little plastic screws that go into the plastic insert to hold the panels together, they can easily be ruined due to the fact that they are plastic. I bought wellnuts off e-bay for a 10-24 or 10-32 screw thread and then used SS screws. You drill a hole small enough for the wellnut to go into, you have to push a little, make sure it goes thru both panels then when you tighten the screw the wellnut expands and keeps them together, best part is they are easily removable and reusable. I also used a couple of them on the mud flaps where it wanted the mud flap to stay against the inner fender. When you put the mud flap up inside the fender you will see a bracket sticking out in the way, I just made a slice in the flap to go around the bracket so the flap goes all the way to the highest point inside the fender. On the drivers side there is an electrical gizmo with relays or something so I made another flap of material and rolled it up and over that electrical box so mud and crap would not fly up there off the tires. Just made the flap long enough to attach to the front of the box and roll up and over the electronics far enough to hold itself up there. The electronics appear to be sealed but did not want to have to use a high pressure spray to clean that area so the added flap helps and it has never unfolded itself. Just something I thought might help a potential problem down the road. I fitted the drivers side flap first and once I was satisfied with it, I just flipped it over and traced it for the passengers side. I have a lot of the mat material left over and have been watching what areas might need added protection but so far nothing else has appeared. I did some mudding on Saturday, we had some unusual warm (60 degree) weather and after all the rain we been having I got the itch! And yes it was muddy, I will probably have to resurface my own trail when it dries out but boy was it fun! Yes that little machine went thru anything I threw at it without a stutter. Bad part was it took an hour of cleaning for 3/4 hour of riding but the mud flaps did their job and protected all those nooks and crannies from accumulating all that mud.