1) +1 rep points... i gotta do this

2) Another trick to keeping those bolts from spinning is a jab of JB-Weld underneath the head (assuming you have some time to wait for it to cure) if you don't have a welder

3) I believe this piece is "hanging" on the frame with all of the bolts in double-shear. Decreasing the size and grade of the bolt will reduce their overall load capacity.... however, if you're just replacing the middle two bolts and retaining the OEM outer two bolts, it's probably not reduced by a significant amount. I'm sure it's OK, especially since these parts tend to have a ridiculous safety factor built in to account for degradation of the bolts & frame for the life of the vehicle, and some planning for stupidity (there's always someone out there that'll grossly overload it). Do you know what size the stock bolts are?