Unfortunately, if you've done what I think you've done, you did get ahead of yourself.

I'll bet that you sheeted the nose, before installing the engine's blind nuts. If so, you can still get to the lower blind nut locations easily, so you won't have a problem, there.

The top location is a different story. You'll need to remove enough sheeting, to give yourself enough room to install the blind nuts.

It's more embarrassing, than hard. Hopefully, the location will not be on the same level as the top ply plate. If it is, you'll need to open up more sheeting, to give yourself room to work, hogging out the ply area where the nuts need to be. When I mounted my Mag 1.60 twin, this was the case. I had the holes drilled, and blind nuts mounted, before I mounted the firewall. This made the problem easy to see, and much easier to correct.

This thread may be of interest , and provide you with a few ideas. http://www.rcgroups.com/forums/showthread.php?t=956429

This one may interest you, too. http://www.rcgroups.com/forums/showthread.php?t=1356049


Like I said, this really isn't hard to correct.