Metal removal at rear, opinions

[Ivan141]

The headlight covers weigh next to nothing, not much to gain from changing those. The bulk of the weight is in the rest of the mechanism.
Maybe you could benefit from some lighter sealed beam units, but I don't see creativity in that department passing scrutiny for a road registered car.
You could change the roofskin out for an FRP version, but again the gain is very little (albeit in the best place to remove weight), while the effort to install a new roof is high.

About the rear diffuser: building an effective diffuser is indeed something best left to capable engineers with a windtunnel (although I gather that the main thing in making them effective is running a very tight ground clearance). But making an undertray type of thing that keeps air out of the general area shouldn't be too difficult to create and test (compare with the same car without the undertray, use a magnehelic pressure differential meter to get solid measurements, if you ever read up on autospeed you'll know what I mean).