I would never trust screws to hold anything in aluminum that was going to be subjected to stress. Drill through the transom and through-bolt it using large fender washers or aluminum plate for backing.
2015 Seaark 1872 MV CC Etec-90
Helix 12 CHIRP SI, HB 1199, MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner