I personally would NOT use screw in anchors. I'd drill a hole through a cross stringer that went across all the stringers in the ceiling crawl space and use a bolt/nut style anchor. Hanging from one anchor for a few seconds doesn't really prove anything. It's the repeated pulling in lowering of the hardtop that will cause those anchors to eventually be pulled loose. For what a hard top window or replacing the hardtop itself would cost a little extra work would be well worth the time and cost of using a bolt style anchor.