That part on eBay is NOT for a 3100. There are two mounting pads on the radiator support for the 3100.
The 3100 had cross bracing rods as shown in the second picture that Whitedog posted. I used "had" since a PO cut them off in my truck. I see that the part in the picture is missing one of the mounts.
Again, check the assembly manual. You can see these things.
