I'd drill through but a smaller size and preserve lathe mount threads (stick some centre marked studs in the thread first and drill through them)? Not sure what size the mounting bolts would be then? If too small for the job...... drill out any way to fit rotary table, then carefully make a back plate with six holes, three to take the new bolts to hold the chuck and three to mount the chuck on the lathe? Or, drill and tap three new mount lathe mount holes in the chuck.
Or, can you get long (1 1/2 inch) bits of t nut profile to fit? Drill and tap the ends. Take the chuck apart and turn a groove 1/8 to 1/4 from the mount end, only needs to be shallow. Make 3 L shaped clamps with a hole in the midle of one side, the other the same length as the groove from the back of the chuck. That end goes in the groove and clamp it down using the t nut overhang?

Tom