Yes, glad you mentioned the fastback, I almost have a solution planned for that (should, if it works be useful for other fastbacks).
There is one of these cars up for sale, with one of the only 3 hardtops I know of in existance:
onyx firecat with hard top | eBay
Look at me posting links!
My fastback would stretch back to the end of the bootlid. Hopefully with a sharper tip/ flat horizontal plane as I have read this is better.
Ok, so regarding front air flow, I need to let go of a lot of air out of the top of the bonnet. I do need plenty of flow.
The rear lower section is going to be the most awkward to sort out but pretty vital. Is there any data (I will google but as yet have not) for adding a rear diffuser and drag reduction achieved? My car is front heavy and light, the addition of a little weight, especially at the rear is no issue at all.
It will gain a front air dam, splitter and rear wing for track handling. The rear beam has been stiffened up and polybushed to give a lot more anti-roll, so the wing is needed to stick it back down at the rear.