If you are going to get in there with a hole saw, you might as well make the hole big enough to use as a clean out. But first try to see if and where any internal baffles are that could interfere with the float arm.
If the glass isn't thick enough to hold a self-tapping screw, cut a backing plate out of a piece of starboard and attach it inside with [dare I say it?] 5200. Mount the fuel
sender in the cover plate and attach the coverplate to the tank with a gasket
and screws. Don't forget that you need some clearance above the sender too.
Oh Yeah! EMPTY THE TANK FIRST, disconnect and plug
the supply line, and clean every shred of glass debris out before you refill it