Well, my suggestion would be to take your last mod back off and see what happens.
And I don't really have much first-hand aero knowledge so take this for what it's worth, but it looks to me like the front undertray guides air under the car, rather than around it, and since you don't have a full belly pan, it looks like the lip of the rear pan would catch the air going under the car.
Instead, might I suggest a front air dam like this (although maybe not QUITE so radical):
The Car
A full bodied 1996 Impala SS that has been in the wind tunnel for
aero mods. Improved the Cd from .320 (stock) to .256
The Engine
A naturally aspirated fuel injected 572 CI Dart big block. 700 RWHP
The Future
Setting a record at Bonneville. There is an existing class record at
241 MPH / 387 KMH. This is a blown gas record that the GIANT SS
could run against. With turbo charging (12 lbs. boost) we can
reach 250 MPH / 402 KMH.
Here's a pic before he took it to the wind tunnel:
I've spoken with John (the owner of the car) in person a couple of times when he brought it to St Louis for the National club meet.