Just a note that you should have a full flat undertray for this car from splitter to diffuser. Here is a GT500 underbody for refreance but keep in mind that this is a decent bit more extreme than what you would see in GT300 particularly the front splitter area.
So, basically i add a flat piece under the driveshaft and the suspension parts and i make the fender parts a bit more closed ?
Pretty Much with something like this for the shape of the floor just make sure that nothing sticks below the floor. It should be the lowest point of the car besides the tires.
Splitter is connected to the undertray but is a separate part normally the split line is at the axle of end of wheel well. Diffuser is the same and would be separated in front of where it starts near the beginning of rear wheel well. Here are some images of the NSX GT3 undertray to show.
There's a little problem, the engine is taking a lot of the space needed to put a diff. roughly the white square.
Aren't your current plans to change the engine position to mid engined anyways? That would make that white box irrelevant as the end of the transmission likely wouldn't go very far past the rear wheels.
So you have 3 options. 1. A lowered center section the minimum width needed to fit the engine and gearbox. Basically take the same curve as the rest of the diffuser but make it start later so it doesn’t interfere and then cut the end short. This is pretty primitive but it works. 2. Group C style. Similar to above but instead of it being a box the lower section shrink wraps around the transaxle. 3. Modern LMP. Center section around the engine and transaxle is flat and forms a teardrop shape in center of diffuser.
Having the diffuser extend way out past the bumper wouldn't be a bad idea: Visually it looks off but, the GT-R has something similar and it managed it with a bit of a 'longtail', fender aero extends past the taillights: The diffuser is basically level with the gurney flap. I like this image that Formula24 shared. Basically two curves, the center portion angles more sharply vs. the outer ones. That could give you the clearance you need for the drivetrain:
