The 4mm steel wing hold down bolts are plenty strong enough for the job. I like to use 1/4" nylon bolts simply because nylon is less likely to vibrate loose. You might go with the supplied bolts and see if they remain tight. If there is any problem, it's no big deal to convert to nylon bolts later. I'm currently putting together the A-K Hellcat, and it provides the same 4mm wing bolts. I went ahead and changed it to nylon, as the blind nuts were not preinstalled. Since I got to do the complete installation, it was no more trouble to go with the nylon. I had some in my "treasure" box, anyway.