Probably 100 ways to put your boat together. The advantage of using enough of the jet ski bottom to include motor mounts is that you don't have to engineer the motor mount positions. The engine needs to be in exact alignment with pump. You will need an alignment rod to check this as you are screwing & gluing the insert down. The insert won't have near the stiffness as it did in entire molded hull & so it would be easy to lock in a warp if you aren't careful. It took me forever to shim the alignment on mine (Polaris).
You can buy alignment rod for about $60 on Ebay or rent one from a guy in Wisc., He is a jet ski aftermarket shop, forget the name.
The pump inlet needs to be the lowest point on the bottom. If you make your own mounts & limit the plastic insert to just the intake & pump that would be a much smaller hole in bottom, which is a good thing.
Not sure of your question regarding attaching to both stringers. I put them on both sides of insert. I notched the cross ribs back so the angle is down tight with hull skin & welded the rib ends to angle. Front of angles make tee joint with unmolested cross rib in front of engine. I cross bolted the insert to the stringers in about 6 places each side. The plastic insert is sealed to hull with 3m5200. The jet pump 'box' is screwed & glued really well to the transom.
Hope this makes sense. I had pictures until my phone went swimming.
