Honestly, instead of tapping the frame, you could use self tappers. I do commercial door installation and we use them a lot on Hollow Metal doors for closers, hinges, etc. They go through a lot of stress and movement in the field and never have I gotten called back for a self tapping screw falling out or anything. They're just easier to deal with. Another idea I used once on a rear fender were metal conduit clamps. They are almost like a U shape with 2 holes in flat tabs on either side. Almost like an Omega symbol..... if that makes any sense, haha. They're used to hold conduit to walls and screw into the walls. Your frame being the "conduit" and then bolts and nuts through your seat.