Trouble mounting case feeder to Hornady LNL AP

I had the LNL AP press mounted with 1/2 inch bolts, but the mounting plate for the case feeder (Part #42) has two holes which measure 0.4"

Why is this so? Should I just use a narrower bolt, and will the press still be held securely? The manual says use 5/16 inch, and according to my measurements a 3/8 should fit (0.4" hole)

Would I be better off drilling the mounting plate so that I can just use the 1/2 inch bolts I already have, or just go down to the 3/8 inch? I've never drilled through sheet metal, and I don't have a drill press, so I'm not sure how I would go about this. What was Hornady thinking when they made two parts, which are supposed to be mounted together, have different diameter holes?

I looked at a 5/16 inch bolt, and it looks skinny and pathetic next to the nice thick 1/2 inch bolts which currently hold the press on. Plus, I already have 1/2 inch holes drilled in the bench, so if I used a narrower bolt, would that be a potential problem? I'm frustrated, I was looking forward to getting everything set up tonight on the case feeder, and now its back to the hardware store.

Has anyone else noticed/dealt with this problem, or is it no big deal to use the smaller bolts? Ugh...

Good to know. Now, the only question is, since my bench (actually a sandwich of plywood and 2x8 screwed together and clamped to my bench) is already drilled for the larger hole, what should I fill the holes with to keep the screws from moving around?

I really, really don't want to have to make a new top and redrill and reattach all my tools (2 presses, trimmer, swager, etc)