[brucepick]

I think Sheepdog is getting at a concern related to mine. If u read a good bit in the aerodynamics sub-forum you will learn that the back end of the car is where serious losses are.

You can clean up the underbody with a belly pan, it doesn't require welding or fiberglass just care and patience. Grill block helps in several ways and is pretty easy unless you require a super-bling look.

However the car's back end sucks a huge vacuum behind it. You create the vacuum using energy created by the engine (= fuel used). The only real solutions we've found are to taper the rear, either a boat tail or a Kamm back which is really a chopped off boat tail. Untreated, it's a major source of drag.

A boat tail doesn't require a chopped top. Chopping would mean less length needed to taper it down to a given size. But chopping also means you're on your own for all the glass, probably won't have wipers, etc.