IMHO, It is best to fit it to the actual hull. Nothing is straight or square on a fiberglass/wood boat.
Locate the engine relative to the pump for driveline angularity. Level the engine side-to-side relative to the gunwales.
Locate and mount the rails relative to the stringers and the engine. In the case of a mechanical fuel pump, make sure that the rails are low enough that the blocks and rails don’t interfere with the pump.
Fit the plates to the rails and stringers then transfer punch the bolt hole and dowel pin locations.
Fit the blocks to the angularity between the plate and rail and locate the holes.
Once you get all of the hole locations, the plate can be trimmed as desired. Make sure that you leave enough material around the mechanical fuel pump window so that the edge distance doesn’t become too thin. Other considerations to motor plate profile might include provisions for belt driven accessory mounts; alternator, vacuum pump, etc.
Cheers,
Joe
