Go to a boat rebuidling/repair/custom shop, they can order the right bracket for the job and install it. The aftermarket brackets are custom made to order, not the Grady Drives.
Since the bracket spreads out the forces, and I expect the bracket ordered for a single F350 to have a wider flange at the transom than usual, the boat doesn't know the diference between twins or a big single or equal total torque/ HP. In other words, the bracket needs to splay out more as it moves towards teh transom from teh motor.
Show this thread to the boat shop.
Make sure a Grady Drive is not ordered.
Just a note, the F350's gear case leg may be partially in the water unless you discuss this with boat rebuilder, then he has to include the necessary bracket fab dimensions to allow the motor to tilt up all the way and not have the cowl hit the transom before T&T is out of range. He also has to make the rear bracket face more vertical of an angle on the bracket than teh Grady Drive is, 5 deg more should do it.
If you are wondering why Grady won't do this, is because Grady wants you to upgrade to a NEW Gulfstream to get the F350 !