New modder

[aerohead]

You might just fab up a cardboard and duct tape nose mockup and see how it shows at the pump.
Conventional wisdom does suggest that it would be hard to see on the radar screen of mpg,however,there are a few extant studies in which reversed streamline bodies were wind tunnel tested.While their drag was significantly higher than 'tail-behind' bodies,when wheels are added the difference diminishes.
It might be possible to get to around Cd 0.13 like this,where you might go below Cd 0.10 with the tail where it belongs.
Just not a lot of data on this sort of thing.
Testing after nightfall might reduce anxiety.

[Superfuelgero]

Best mpg is going to be with the stock exhaust. Anything larger and its going to stagnate at our rpms. Tuck and rotate it as needed for the belly pan.