Swim Deck Mounting Guidance

Hello all.  I am a little uncertain, and would appreciate some guidance.  I have a 75 Southwind 20 I am mounting a swim deck on.  It has never had one before so this is a new installation.  I have 4 mounting brackets.  The the two inner brackets are no problem because I can see where they come through the transom on the inside of the boat.  So I can bolt and nut them in place.  My question is for the two outer brackets.  I do not have access to where they come through on the inside of the hull.  Can someone tell me best mounting practices to follow there?  Is there danger in hitting anything there, do I just mount with some heavy duty course thread screws?   Any help would be great.  I'm ready to install, but holding off until I hear back from some wisdom.

Thank you all for the continued help.