Nicely done ! Yep, you (probably) went from a stamped frame speaker to a cast frame speaker, which has significantly thicker metal, and requires a longer screw (or stud in your original configuration). If you have not done so already, I suggest you use a lock washer under the speaker mounting screw, either a split-ring spring washer or a star washer. This will help keep the screw from coming loose under all that vibration. I don't know what size t-nut and screw you used, but I suggest 1/4"-20 (1/4 in dia, 20 threads/inch) for this application (assuming it will fit thru the hole in the speaker basket). If you used #10-24 or #10-32 it will still be fine. Nice work, I appreciate the amount of time and effort it took to do that. Enjoy !!