Drill one hole to finish size and use one mounting bolt + nut to bolt up padeye and backing plate. Align padeye and plate to each other as close as you can by eye or bubble level, square, etc. and tighten bolt.

Now drill diagonal hole, but using two pilot drills as follows: 1/8" drill half way through from both sides until you feel the two holes "meet" in the middle. Follow up with a 7/32" , also from both sides, then push it all the way through. This second pilot is to correct any misalignment in the 1/8" hole and to confirm on-center location. If hole location is slightly off you still have material to correct, either by "walking" the drill or with a rasp cutter in a Dremel. When location looks good, finish up with the 5/16". Install another mounting bolt and tighten. Drill 3d and 4th holes as above. Good luck.