My guess is that they're II lowers on TRS uppers. Look for 2 vent holes on the back of the uppers. If they went to IIs from TRS without recutting the transom, the X dimension will be 2" too low.
Velvet drives will have a tag on the top left (if I remember right). The Merc and B/W look entirely different. I think you can find diagrams for both in Doller Marines online parts listing.
The Merc tranny was not built to handle much hp, I doubt they would have ever built a SSM/Merc tranny combo.
Might be worth buying as a cheap project boat.